
This month, we are sticking with the top wedge of the Aquaponics Cycle, the fish.
Like an older sibling who’s the only one tall enough to reach the bowl of cookie dough on the counter, the fish in aquaponics systems get the first crack at the nutrients in fish feed. The amount of cookie dough that gets shared with the younger siblings depends on both the nature of the older siblings (are they the type to share?), and the type of cookie dough in the bowl (is it their favourite, or do they hate it?). Similarly, in aquaponics, the nature of the fish and the nature of the feed dictates how many nutrients will be shared with the plants and the microbes. In my last blog, we focused on nitrogen. For this post, we are going to talk about phosphorus.
Phosphorus is important in aquaponics because it’s an essential macronutrient for all three types of organisms in our systems: the fish, the microbes, and the plants. In all three, phosphorus is a critical component of the molecular instructions that store the cells’ operating instructions (DNA), as well as the powerhouse molecules the cells use for energy to get things done (ATP). In microbes, phosphorus is important for the formation of strong cell membranes that protect the microbe from its environment. In fish, phosphorus is essential for building and maintaining strong and healthy bones, and ensuring efficient growth. In plants, phosphorus plays an important role in flower and fruit formation; without it, the yield and quality of your peppers, tomatoes, or cannabis will drop dramatically.
Almost all the phosphorus entering an aquaponics system comes in from the fish food. And the phosphorus in that food comes from a lot of different sources, generally fitting into five categories: phosphorus from animal bones (Bone-P), bound phosphorus from plants (Phytate-P), other types of organic phosphorus from plants or animals (Organic-P), and 2 types of inorganic phosphorus supplements (Monobasic-P and Dibasic-P). See chart.
Fish | Bone-P | Phytate-P | Organic-P | Mono-P | Di-P |
Carnivore | 68% | 0% | 84% | 89% | 64% |
Omnivore with stomach | 75% | 27% | 95% | 93% | 62% |
Omnivore without stomach | 0% | 0% | 72% | 86% | 30% |
The digestibility of an ingredient is a measure of how much of that ingredient stays inside the animal that eats it. If a source of phosphorus was 100% digestible, the fish would eat it, and there would be none left in their feces. If it was 50% digestible, half would stay in the fish, half would end up in the feces. If it was 0% digestible, all the phosphorus would go in one end and out the other. The source of phosphorus, and how tightly bound in a chemical structure it is, impacts how digestible it is.
Now this is where things get interesting and where aquaponics splits off from traditional aquaculture. In traditional aquaculture, you want the 100% digestible source, the greedy sibling who won’t share any cookie dough. You want all the phosphorus to stay in the fish because otherwise it can get out into surrounding waterways, fueling out of control algae growth and eutrophication. But in aquaponics, if 100% of the phosphorus is in the fish, there’s none left for the microbes and the plants. In aquaponics you want your fish feed to have enough digestible phosphorus to grow your fish, with some indigestible phosphorus leaving your fish to grow your plants/microbes.
So, what type of phosphorus do you use for that? Well, the complicated part is, that depends on what type of fish you’re using. The exact same ingredient in 3 different fish will behave very differently.
The diagram below shows three different species of fish: rainbow trout are carnivores, they have an acidic stomach and a short digestive tract; catfish/tilapia are omnivores with an acidic stomach and a mid-length digestive tract; carp/koi are omnivores with no stomach, but a long, winding digestive tract. Our fish with stomachs digest more Bone-P and dibasic phosphorus – both of which have tightly bound molecules of phosphorus. Why? Because the acidic pH of their stomachs helps dissolve the molecules, allowing phosphorus to be absorbed into the fish. Phytate isn’t easily dissolved — it needs a special enzyme to break it down, so it isn’t available to carp or rainbow trout (in one end and out the other), and only marginally available to tilapia. And compared to the other two species, carp are just pretty bad at digesting phosphorus (they’re the naturally generous sibling).

Illustration by Ross, based on this reference. Digestibility estimates from here reported for rainbow trout, tilapia, and carp.
Let’s assume that you’ve looked at your feed ingredients, and you’ve matched them up with your fish’s digestive system, so you have a good mix of digestible and less-digestible phosphorus. You’re left with fish poop that has a nice amount of phosphorus in it, just enough to grow your microbes and plants. But there’s still a problem. That phosphorus is still bound to other molecules, and is stuck in the poop. So what now?
In my next blog, I’ll discuss the zoo of microbes that are key to freeing this phosphorus.
