If they're meat plants. According to this article (http://animal-world.com/encyclo/fresh/Knifefish/BlackGhostKn...), they're carnivores who can eat pellets. Reading this entire page, it looks like Black Ghost Knifefish are not trivial to keep happy and healthy, and will eventually need 100 gallons of water. While I would argue that it's irresponsible to treat any fish as a mere fertilizer producer, this fish may be especially miserable for its limited life.

Hmm, is there some other kind of fish better suited to this type of system? Some kind of fish that could naturally survive on the plant waste? Maybe not even the waste in the water but some other form of biomass from the rest of the system?

e: Sibling comments seem to suggest Tilapia or Rainbow Trout.

Not sure what you're proposing, but something has to go into the system (fish food or fertilizer) if you're going to be taking things out (food). If the fish are living off of the plants, then you need to feed the plants. If the plants are living off of the fish, then you need to feed the fish. Something something Law of Conservation of Mass and Energy something something.

Yeah good point, that must be why all ecosystems fail without human intervention.

/s

In a natural ecosystem, nothing is taken out, whereas in this one, food is supposed to be removed from the system, making inputs necessary.

Sunlight and air are inputs though. Most of what you take our of the system comes from changing H2O and CO2 from the air into a different form using energy from the sun.

There is the potential various micro nutrients, and minerals. However the vast majority of what the system needs is air and sunlight.

Right, but nutrients and minerals are still being regularly removed from the system--they need to be put back in order for the system to function. Their mass relative to the mass of other inputs doesn't change the calculus.

Right. Just ecosystems wherein humans regularly remove a considerable portion of the overall biomass. Like a garden.

Yeah conservation of matter, you need minerals other than carbon, oxygen and sunlight to make food therefore if human in the loop is introduced you need to take the human byproducts into the system as well

Why do you feel that using a fish for fertiliser production is irresponsible? It seems to be an effective way of using something that the majority of people who keep fish treat simply as a waste product.

I use the water from my tank's water change to water all my plants in the house. I can fill almost three watering cans which is enough for two waterings of the various decorative plants and herbs I have (I don't have any vegatables or this surely wouldn't be enough). And that's just a measly 10 gallon aquarium.

His Black Ghosts are in a 150-gallon tank. They'll be quite happy.