Media in Aquaponics

Correctly choosing your medium/media (media is the plural form of medium) is perhaps the greatest challenge any aquapon* can face in the making of an aquaponic system. The medium will determine your success, among some other factors. Too much carbonate buffer, and the pH is too high, too low, and your pH will wildly fluctuate. In fact, the single largest cause for failure in an aquaponic system is the choice of a high carbonate medium. There are two, productive, things you can do, 1) stop growing plants and filter the water with a mechanical filter or 2) replace your medium. Both of these are incredibly expensive--on the order of hundreds of dollars (USD) for a recommended sized system (1000L for the fish tank and equal volume for the growbed). This will depend on other factors, but it is generally that expensive. This is a problem that can easily avoided, however.

Firstly, though, let's understand some of the water chemistry happening in an aquaponic system. The pH of a system is recommended at 6-7. This differs from the recommended pH for a hydroponic system (5.5-6) because aquaponics is a living system. In aquaponics, the pH must be a compromise between fish, plants, and bacteria. Bacteria and fish prefer slightly basic conditions, while plants prefer more acidic conditions. Nitrates, Nitrites, Ammonia/Ammonium are also present. These are measured in a unit called Parts Per Million or PPM, which is equivalent to mg/L. Nitrates are taken by plants as usable Nitrogen for proteins, so a sign of a healthy system (well-balanced) is a Nitrate level of around, but over, 0mg/L, but Nitrates can safely rise to well over 500mg/L for tilapia species.   However, ammonia, ammonium, and nitrites should never rise beyond around 5mg/L for the sake of the fish. Even then, it's a crisis in your bacteria colony. 

The pH, however, will be what is dealt with most directly in a choice of medium. A high carbonate medium, limestone and marble, will raise your system's pH to a level that "locks out" iron for plant use. The cause for this stability in pH (at a high pH) is something called a "buffer". A buffer is a conjugate acid base pair, such as H2CO3 and HCO3-. The first is Carbonic Acid--the acid responsible for ocean acidification as CO2 levels rise in the atmosphere. Carbonic Acid is in a equilibrium with the bicarbonate ion. This means the levels don't change between them, even as the reaction is happening (the rates of reaction between them are the same). Carbonic Acid is produced by fish breathing out CO2 through their gills. This CO2 gets dissolved and turns into Carbonic Acid. When you add excess CO3 or HCO3 in your medium, you are causing the reaction to deplete the reserves of hydronium ions (H3O-) which is responsible (along with OH-) for changes in pH. Therefore, it will take a lot of H+ (produced by your bacteria) to get rid of all that base. In the meantime, your pH will be nearly stable--over 7 for this substance. 

This is, in fact, the reason why most systems that end up failing fail. There's nothing you can do about it other than the options above. However, there is a way to prevent this from happening to you, and it's simple! You could pay for a high-quality, inert, medium such as clay pebbles, expanded shale, etc.; or you can do the "fizz-test". First pointed out to me by TCLynx on the Aquaponics Community , it's a way of determining whether the gravel you are buying has carbonates in it--pour a cheep, weak acid on it. Usually, acetic-acid (vinegar) is used, because it's cheep and can be found anywhere. I personally think HCl would be more fun, but to each their own. If it, well, fizzes, then DON'T buy it. If it stays inert, feel free to order it--gravel is usually cheaper than other mediums.

In my opinion, gravel and water is the best media combination. One gravel bed and a few deep water culture grow-beds. But, that truly is a point of prejudice, not scientific fact.

Happy Gardening!--Blackberries are for Picking

*meaning someone who practices aquaponics