|The two kinds of snails in my garden. Malaysian trumpet snail|
on the left, Rams horn snail on the right.
In soil gardening, worms are particularly useful. They perform a number of benefits to the soil. In an aquaponics system, snails can be just as useful to the water ecosystem as worms are to the soil, if not more so. They perform a number of beneficial functions in the tank. First and foremost, they are the clean up crew. If you overfeed your fish, your snails will gobble up all that extra food in short order. If a fish dies and you don't notice, the snails will jump on it and gobble up the remains in just a few days, lessening the impact to the water chemistry. They also act as an extra food source for the fish, particularly if you have tilapia. The tilapia will eat the little ones whole and will eventually figure out how to pick the shells apart on the big ones to pluck out the juicy bits.
In fact, because of these two facts, snails end up being a useful indicator of how accurately you are feeding your fish, which can be particularly useful if you feed them sinking food pellets. If you are overfeeding your fish, the snails will latch onto the ready food supply and increase in numbers rapidly, multiplying in just a couple of weeks. If you are underfeeding your fish, the fish will turn to the snails as a source of food, and the snail population will plummet. They are even a good indicator of water quality. If you have a water quality problem, perhaps caused by a big rotten fish, low oxygen levels, or some other problem, the snails will attempt to escape the water. So if you come check on your tank and there is a big line of snails right at the water's edge, you know you have something to fix.
Another useful function is the role they play in cycling minerals, particularly calcium. If your water supply is hard, like mine is, there are lots of calcium salts in the water. In aquaponics, this can be particularly tricky. Water is added, bringing with it more salt for every gallon. But the water leaves the garden via evaporation, leaving the calcium behind. The snails make their shells by pulling the calcium out of the water. When they die, or are eaten, that little calcium pellet falls to the bottom of the tank. If you run your tank for several years, the snail shells build up. If calcium is short, the living snails will get their calcium from the dead snail shells. If there is plenty, they just pile up. I love scooping the old shells out and adding them to my garden soil. They act as a slow release calcium supply in the soil.
The question is, what kind of snails do you want? First of all, don't get the ornamental snails. A lot of those are considered beneficial in the hobby aquarium because they don't reproduce rapidly. That's actually a problem in aquaponics. You want lots of snails. There are two main kinds of snails, Malaysian trumpet snails and rams horn snails. The trumpet snails have a long curl to their shells and are shaped like an elongated cone. The rams horn snails curl outwards and don't form any kind of a trumpet shape. There are a couple of other kinds of pond snails out there as well that have an intermediate cone shape.
The Malaysian trumpet snails can be useful in their own way. The shell shape helps them burrow through sediments, where they prefer to live and eat. In their burrowing, the perform much the same function as the earthworms do in soil, they aerate the sediment and help eliminate anaerobic patches which can become smelly. However, their shell is too hard for the tilapia to eat. They can still be useful, though. As they overpopulate, just scoop them up and toss them in the soil. They are pretty tough, but they can't move around outside the water and only survive a couple of days. Or, if, like me, you have a pet turtle, you can just use them as supplemental turtle food. I have a little 3 stripe mud turtle. His beak has no trouble crushing the hard shell of the trumpet snail and he considers the long shape to be perfectly bite sized.
The rams horn snails are the preferred snail in aquaponics. I have had a few of the medium conical snails and they work well, too, but don't thrive and reproduce as well as the rams horn snails. The primary food source of the rams horn snails is algae, extra food, and whatever other organic debris ends up in the tank. They can get up to a half inch in diameter, though that is rare. The few I have gotten that big always earn the name "Monstro the Snail" from me. Usually they are much smaller, maybe a quarter of an inch in diameter.
So I hope you consider adding snails to your aquaponics garden. I know I love what they do for mine.