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Water chemistry for aquaponics

“We forget that the water cycle and the life cycle are one.”

Jacques Yves Cousteau

cat fish being weigh from backyard aquaponics

Water Chemistry in aquaponics

Let’s speak about what I feel is most common problem with maintaining good growth in aquaponic systems. Water quality this usually comes down to the utilization of (Dissolved Oxygen). This is, supply, exchange, and replenish.

Water is classified as hard or soft, hard water contains much more dissolved minerals than soft water does. Hard water forms over time when these minerals are absorbed by groundwater. Calcium and magnesium are the two larger atoms in your water/solution and therefore the most conductive minerals/elements of all.
Why should I learn about water chemistry?
The next step would be for you is to understand what that means to you as a hydroponics grower in a long term recirculating system such as the Bio-Buckets. Hard will usually have a high pH but not necessarily, this will depend on the alkalinity of your water source, this is the reason I recommend using tap-water because it has been treated to have the most stable levels of alkalinity and by using any form of water filtration system you have there by destabilized your water alkalinity levels among other things, and thereby will not hold up under long term recalculating growing conditions, but on the other head great for drinking water but were not trying to produce good drinking water for your Bio-System, were trying to produce the best stabilized water possible and the alkalinity levels tell you what that is……..are we begging to see the light yet? Well if not, don’t worry were not done yet!!!
The obvious problem for the grower is that he will be adding quite large amounts of acid on a regular basis. If using Phosphoric acid this may lead to a build up of Phosphate in the reservoir over time. High levels of P in the solution can inhibit the uptake of other salts, Zinc for instance, and cause general nutrient imbalance.
The first and most obvious solution is to change-out or flush regularly. This will reduce the chances of Phosphate accumulation and ensure maintenance of a good nutrient profile. Frequency of changes-outs or flushes truly depend on the volume of water/nutrient reservoir size and number of plants. In very Hard water arias however a large amount of Phosphoric acid will be needed to correct (pH) when nutrient is first made up.

live barrer fish and live plants

How does this affect my aquaponic system?

The ultimate determining factor in the success of any aquaponic system is water chemistry, for that is the life force of any aquaponics system.
What You Need to Know About Water Chemistry, and Why?
Water has four measurable properties that are commonly used to characterize its chemistry. It is a must that we explore these four properties if we are to determine the health or lack thereof of our aquaponic systems. They are

  • (pH), buffering capacity,
  • (GH), general hardness,
  • (KH), calcium carbonate/ (dH), degrees of hardness and
  • (Salinity)


Put simply pH is a measure of acidity. The pH scale goes from 1.0 which is highly acidic, through to 7.0 which is neutral, up to 14.0 which is highly alkaline, but really what does all that mean?

Two aspects of pH are important. First, rapid changes in pH are stressful to plants and should be avoided. Changing the pH by more than .5 units per day is known to stress plants. Thus, you want the pH to remain constant and stable over the long haul. Second, plants have adapted themselves over time to like a sustain pH rage between (5.5 – 7.0). your job as a grower is to do your best to keep it in-between those numbers for best results.
Most plants can adjust to a pH somewhat outside of their optimal range. If your water’s pH is naturally within the range of 6.5 to 7.5.

healthy fish gills

Buffering Capacity (KH, Alkalinity)

Buffering capacity refers to your systems water’s ability to keep the pH stable as nutrients or additives are added. pH and buffering capacity are intertwined with one another; if the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the “sponge” absorbs the acid without changing the pH much. The “sponge’s” capacity is limited however; once the buffering capacity is used up in your system water/nutrient, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). I feel I need interject some here, remember those little things I talk about all the time you know (Beneficial Bacterium) they are responsible for accelerating nitrogen cycle and producing nitric acid that is (nitrate), right about now there’s another one of those little lights going off in a growers mind. Without buffering, your tank’s pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing), is this all ringing any bell’s or what!! On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your plants, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Attempts to change the pH of water usually fail because buffering effects are ignored.

How much buffering does your Bio-System need?

The larger the (KH), the more resistant to pH changes your water will be. Water (KH) should be high enough to prevent large pH swings over time. If your (KH) is below roughly 5.0, you should pay special attention to your tank’s pH (test daily, until you get a feel for how stable the pH is). This is ESPECIALLY important if you neglect to do frequent partial water changes. In particular, the nitrogen cycle creates a tendency for an established systems pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the (KH). If your pH drops more than roughly two tenths of a point over a day or two, you should consider increasing the (KH) or performing partial water changes more frequently. (KH) doesn’t affect the plants directly, so there’s no need in immediate action but I would keep an eye on it.

Pacu fish from backyard aquaponic system

What are the best ranges for plants and fish

General Hardness (GH)
General hardness (GH) refers to the dissolved concentration of magnesium and calcium ions. When it is said that some plants prefer “soft” or “hard” water, it is (GH) (not KH) that is being referred to.
It Should Be Note: That (GH), (KH) and (pH) Although as different as they are all three properties are distinct, they all interact with each other to varying degrees, making it difficult to adjust one without impacting the other. That is just one reasons why that BigToke recommends that beginner hydro-newbie’s are advised NOT to tamper with these parameters unless absolutely necessary, or under the direct supervision of a mentor or a very experienced grower who understands the basic properties of water chemistry. As an example, “hard” water frequently often comes from limestone aquifers. Limestone contains calcium carbonate, which when dissolved in water increases both the (GH) (from calcium) and (KH) (from carbonate) components. Increasing the (KH) component also usually increases pH as well. Conceptually, the (KH) acts as a “sponge” absorbing the acid present in the water, raising the water’s (pH).
Water hardness follows the following guidelines. The unit (dH) means “degree hardness”, while (ppm) means “parts per million”, which is roughly equivalent to mg/L in water. 1 unit dH equals 17.8 ppm.
General Hardness

0 – 4 dH, 0 – 70 ppm : very soft
4 – 8 dH, 70 – 140 ppm : soft
8 – 12 dH, 140 – 210 ppm : medium hard
12 – 18 dH, 210 – 320 ppm : fairly hard
18 – 30 dH, 320 – 530 ppm : hard
higher : liquid rock (Lake Malawi and Los Angeles, CA)

Did you folks know that by measuring the salinity of your hydroponics systems you can get the total amount of dissolved substances. Salinity measurements count both (GH) and (KH) components as well as such other substances as sodium. Salinity is usually expressed in terms of its specific gravity, the ratio of a solution’s weight to weight of an equal volume. One component of salinity that neither GH or KH includes is sodium. Is knowing your Bio-Systems water’s salinity very important in nutrient management and long term use? Is knowing (pH), (GH) and (KH) suffices important to any hydro-grower? I will discuss this at a latter time but for now I made up something for you, this is a basic 3D representation of what I am trying to say, once you understand the makeup of basic water chemistry you will have a better understanding of what’s happening in your system.

close up of water letuces

Nutrients and Trace Elements
In addition to (GH), (KH), (pH) and salinity, there are a few other substances you may want to know about. Most tap water contains an assortment of nutrients and trace elements in very low concentrations. The presence (or absence) of trace elements can be important in some situations, specifically: [list][*]nitrates, which are in direct conjunction with the NITROGEN CYCLE [*]phosphates, the second most prominent nutrient. Phosphates have been linked to algae growth. If you have persistent algae problems, high phosphates may be a contributing factor, let me say something here, I do not believe that the lucas formula for the Bio-Buckets use it at your own risk. To control algae, only if not using the Bio-Bucket System, frequent partial water changes are often recommended to reduce nutrient levels. If growing in a Bio-Bucket System that I have laid out, there will be no problems for you because the Beneficial Bacterium will control all algae.

Altering Your Water’s Chemistry

Hardening Your Water (Raising GH and/or KH)
The following measurements are approximate; Note that if your water is extremely soft to begin with (1 degree KH or less), you may get a drastic change in pH as the buffer is added.
To raise both (GH) and (KH) simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the (KH) and (GH) by about 1-2 (dH) degrees of hardness.
Did you know….the (KH) calcium carbonate and the (dH) degrees of hardness thereof, are the determining factors of your waters buffering capability’s, in other words let’s say that your (pH) to high, and you have to add LOT’S of ph-up to get it back to normal, that would tell me that your waters degrees of hardness which is calcium carbonate is very low!! But if you only had to add a little amount of ph-up that would mean that you (KH) plus (dH) are not to bad off……whoops I hear another one of those little bells going off!!!
To raise the (KH) without raising the (GH), add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the (KH) by about 1 (dH). Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.

Raising and Lowering pH

One can raise or lower (pH) by adding chemicals. Because of buffering, however, the process is difficult to get right. Increasing or decreasing the pH (in a stable way) actually involves changing the (KH). The most common approach is to add a buffer whose equilibrium holds the pH at the desired value. This was what I was talking about of the Bio-Buckets earlier when I said: that if a hydro-system that did not build upon it’s self, then it could not sustain life. Did you know….I’ll bet you didn’t know that the Bio-Bucket System is built around these four basic principals and the support thereof, and that is the (GH), (KH), (pH) and (Salinity).
Note that the exact amount of quantity needed for (pH) up or down depends on the water’s buffering capacity. In effect, you add enough acid to use up all the buffering capacity. Once this has been done, decreasing the (pH) is easy. However, it should be noted that the resultant (lower-pH) water has much less (KH) buffering than it did before, making it more susceptible to (pH) swings when (for instance) nitrate levels rise. Warning: It goes without saying that acids are VERY dangerous! Do not use this approach unless you know what you are doing, and you should treat the water BEFORE adding it to your hydro-system, this is just another reason that I recommend tap-water if possible.
Products such as “pH-Down” are often based on a phosphoric acid buffer. Phosphoric acid tends to keep the (pH) at roughly 6.5, depending on how much you use. Unfortunately, use of phosphoric acid has the BIG side effect of raising the phosphate level in your hydro-system, stimulating algae growth. It is difficult to control algae growth in a hydro-system with elevated phosphate levels.
Did you know…..that one safe way to lower (pH) WITHOUT adjusting (KH) is to bubble CO2 (carbon dioxide) through your hydro-system. The CO2 dissolves in water, and some of it forms carbonic acid. The formation of acid lowers the (pH). Of course, in order for this approach to be practical, a steady source of CO2 bubbles is needed to hold the (pH) in place. As soon as the CO2 is gone, the (pH) bounces back to its previous value. The high cost of a CO2 injection system makes it non practical for hydro use as a (pH) lowering technique. (for inexpensive do-it-yourself alternatives). CO2 injection systems can be found almost every were on the internet because the additional CO2 stimulates plant growth.

Softening Your Water (lowering GH)

Soft Water
“Soft water” is a relative term, but water to be soft must contain low amounts of dissolved calcium and magnesium, which cause water to be hard.
Milligrams per Liter Grains per Gallon

Soft 0 to 60 mg/l 0 to 3.5 gpg
Moderate 61 to 120 mg/l 3.5 to 7 gpg
Hard 121 to 180 mg/l 7 to 10.5 gpg
Very Hard over 180 mg/l over 10.5 gpg

Hard water can also be softened by diluting it with distilled water or R/O water. R/O (reverse-osmosis) water is purified water made by a R/O unit. Unfortunately, R/O units are too expensive ($100-$500) for most hobbyists, but distilled water can also be almost purchased at any stores, but for most folks the expense and hassle are not worth it.

Root-to-Root Travel of the Beneficial Bacterium
Bacillus Subtilis GB03 is a Beneficial Bacterium with activity against water-borne fungal root pathogens.

Most people have the attitude that microbes are all harmful, but in fact, most organisms in soil or in solution are beneficial for plant growth. Modern agriculture developed the view that all disease-causing and pest organisms need to be killed, and so the kill-everything-but-the-plant attitude came about. Unaware that healthy soil or solution in fact should contain more beneficial bacterium, and so a program to wipe out life in soil and solution was initiated. But more and more toxic chemicals have had to be used as the diseases and pests develop resistance with the ever-increasing use of killing agents.

Why don’t the beneficial organisms develop resistance to the toxic chemicals being used?

Because almost by definition, disease organisms and pests have a boom-and-bust life cycle, so when one pest organism survives the chemical onslaught, hundreds, or thousands, or billions of offspring are produced. Beneficial organisms rarely employ that kind of growth strategy, but instead reproduce only a few times a year, with perhaps only a few offspring produced each time. Thus, when you use toxic chemicals to control diseases, but in fact kill most of the beneficial organisms in the soil or solution, it takes a long time for the beneficials to return. Thus the likelihood that they will develop resistance is significantly less than that for any disease or pest organism.
Modern agriculture has set the stage for non-stop, never-ending reliance on chemicals. That’s great if you want to sell chemicals, not so great if you need to have drinkable water.
Do we have to go this route? What we need in production agriculture is to help the beneficials more than the diseases and pests. We need to tip the balance in favor of the good guys. What conditions favor the good guys? Do we really need to know all the names of all the organisms in soil or solution, or do we just need to know which conditions favor the beneficials and which favor the diseases?

One is the existence of an element

the other is the effect the chemistry of an imbalanced solution has on the availability of that element.
An aged solution’s imbalance can be such that it either has an insufficient quantity of an element existing in the mix, or that the imbalance has changed other properties of the solution to cause the element to become unavailable to the plants. For example, a solution may have had all but a trace of its nitrogen depleted, or it may still contain adequate nitrogen but it will be unavailable because of the pH shift resulting from the imbalance. Either condition is unfavorable to plant health. The difference being that the former points to a spent solution that has no more useful life and needs to be replaced, and the latter points to a solution which may still be useful but is starting to require more maintenance than desired. Although both points may carry merit, this has been my experience in the bio-bucket system, the reservoir is designed with a float valve, which is constantly adding fresh water back into the bio-system:

Solution Maintenance Required to Insure Plant Health
Although a solution may pose no potential threat to plant health, most growers consider a solution no longer useful when it causes the grower to spend more time maintaining it than is desired. My Bio-Buckets are outfitted with a float valve, which continuously supply’s the system with fresh water, (tap-water, from cold line) not solution. As the fresh water dilutes the solution in the system the ppm’s go down, and about every other day (depends on stage of growth) just add your nutrients at the desired ratio to bring the ppm’s back up to the desired level, as you (add back) the fresh nutrients to the diluted mix which is already in your system, all you are doing is simply refreshing freshly diluted mix, and bringing that diluted mix back to it’s desired level of ppm’s.
Because of the plants’ relatively higher absorption of water than of salts in the water, maintaining the solution volume is essential in a recycling system in order to prevent salts from over-accumulating in the solution.
Flush the entire system, and replace with fresh solution. This will not be a problem if outfitted with drain valves. A fresh solution has a pH behavior that’s generally predictable, it will fluctuate but will do so within the acceptable range, thus requiring no adjustments or maintenance other than add backs. An older solution finds the pH wanting to run out of range (usually in the direction of the source water pH), this runaway pH drift constantly needs attention. At this point pH can start to become more trouble to maintain than the trouble it takes to replace the solution and return to the lower maintenance of a balanced and well buffered fresh solution. The problem here is that by the time a grower realizes he’s been going in circles chasing a pH ghost, the solution can have already passed its useful life in other respects.
Or, have a plain ready that would replenishes the aged solution? Or not, I have found that it pays to use mathematics rather than guess work when it comes to the useful life of your solution.

A common rule-of-thumb estimate of water usage in a greenhouse is about 1 liter/sq ft/day for vine crops such as tomatoes. It has been my experience in my bio-bucket system, that in-between maximum/minimum of water/solution uptake, (this is not a static time frame,) for a mature indoor garden under strong artificial HID lighting is about (1qt, US Gallons) per plant.

juvenile mixed tilapia in tank

In case you haven’t noticed, the determining factors behind a reservoir’s useful life can all be traced back to the rate of water uptake, which is directly tied to the current demands of the crop. These demands will constantly increase as plants slowly fill their allotted space, often taking sixty or more days and spanning multiple growth stages before peak water uptake is eventually seen by the reservoir for the first time. As more water is being used by the plants, more nutrients are being removed from the nutrient solution, this naturally affects the nutrient balance in the remaining solution. In essence, the nutrient balance is also being controlled by the rate of water uptake. Simply put, a fuller garden space uses more nutrients because it uses more water. So what we have here is a direct relationship between solution volume maintenance (add backs) and pH/TDS maintenance. When that relationship is recognized, and this strategy enhanced to take advantage of it, additional gains in labor can be realized.
Reservoir Sizing, to buffer ph and nutrient uptake

Time Based—Management Strategy
The “replace it every week or two” idea is usually safe regarding plant health, however, it doesn’t distinguish between those using small reservoirs with large crops and those using large reservoirs with small crops. What really determines solution life is the plants’ ability to transpire, which is a function of its leaves. This means that if you have one more leaf today than you did yesterday, that today you would need a little more water than you did yesterday because of the new growth that was born since yesterday. As you can see, water uptake is a constantly moving target, and while it does have an element of time associated with it, it’s really controlled by the mass of leaves in a garden at any given time.


Equal parts technology enthusiast, carpenter, kitchen designer, farmer, capitalist, domesticated father and tree-hugger, I dissected and negotiated myself into a life that brings me both pleasure and pain upon coming to the realization that I cannot save mother earth from Human nature and my only recourse is to do more good than harm through Aquaponics. All of my work can be found here in these 12 main and the supporting pages I use to valiantly put forth this hypothesis.

Damian HInkson