One of those things that you will eventually encounter in aquaponics is fish poop. While the short term problem with fish is that they “exhale” ammonia which needs to be nitrified or diluted to avoid a fish-kill, their solids waste is the long term problem.
Now, fish solid waste (FSW) is generally pretty benign to us humans. Since fish are cold blooded, E.Coli et al are not threats. However, FSW that accumulates at the bottom of a well-stirred tank begins to aerobically decompose fairly rapidly. This is a double-hammer to the fish; this processes steal dissolved oxygen (DO) from the water, and release complex volatile organic compounds. Not enough DO or too much VOC in a fish tank is a fish kill in the making.
If you lose the fish, then it’s double hammer for *you* as the aquaponics gardener. Firstly, it’s a potentially sizeable financial impact — around here, 40 Hawaiian Gold tilapia fingerlings will set you back $400 CAD. Secondly, without fish to produce ammonia, you risk having your nitrobacter(1, see Linkliography below) colonies starve to death. That will in turn starve your plants for nitrate foods.
This is what is called a “system crash” in aquaponics; even in a non-commercial system, the effects can set you back dozens to hundreds of hours and dollars.
There are two ways of dealing with solid waste in an aquaponics system. Those are filtration, and mineralization. For anyone familiar with aquaculture (2, see Linkliography below), filtration is the common answer. In this approach, you use some mechanical process to separate the FSW out of the water it is floating in and then remove it from the system entirely. This avoids the problem of possible decomposition in-tank entirely.
As a word to the wise, just because filter it out doesn’t mean it is garbage. FSW is chock-full of all kinds of great nutrients(3) that a conventional soil-based garden would love to have added to it. FSW, after all, is manure. So, using it as a top-dress for your garden or “green dope” for your compost pile is hardly a mistake for most folks.
The other way to deal with FSW, as I said, is mineralization(4). This is a process where the material is allowed to flow into the gravel grow beds and be washed down through the rock. As this happens, it is repeatedly and heavily oxygenated by the ebb-and-flow process. Essentially, the grow beds are used as mechanical filters that also encourage decomposition to happen where it’s best for the fish and the plants; away from the fish, and close to the plant root structure.
Note that if you are using a constant-flow grow bed instead of ebb-and-flow, this will NOT work. You’ll just be moving the problem around. The process is dependant on good oxygen levels available to accelerate the decomposition and conversion into valuable minerals that are biologically “available” to the plants.
Another issue is grow bed volume. You should have as much volume in gravel, as a *minimum* as you do have water in the fish tank. I have seen some systems go two, three or four times higher than this. However, you cannot go lower; you won’t get enough “work done” by the beneficial bacteria in the grow beds to ensure water quality for your fish.
My in-basement system is about 20 cm / 8 inches deep in the two grow beds. That is about as shallow as you ever want to go. 30 cm / 12 in is better. 45 cm / 18 inches seems to be ridiculously effective for mineralization based on some things I have seen online(5).
Earthworms — specifically “Red Wigglers” — are living bio-reactors. They are amazingly effective at accelerating FSW decomposition, de-silting grow beds, and converting macro and micro-nutrient compounds from being biologically “unavailable” to the plants to being “available”. So, having earthworms in your grow beds is great idea. You can either dig some up if you know what you are looking for, or order a pound of them to be shipped to you from reputable online vendors(6).
Yes, you read that right … you can order worms online(6). Welcome to the “InterWebz”.
I added a few earthworms to both the grow beds of my basement aquaponics system. It’s only been a month or so, but the last time I was digging a hole to transplant a herb, I actually found one of them still happily crawling around. So that’s good news.
That’s it for this article! As always, comments and suggestions are welcome!