Great point. I'll make a quick list right here.
Plants:
Seaweed - produces food under water and reproduces from extra seeds.
Bush - produces food above water, the food is more caloric than seaweed food. The seeds may fall into water. Bushes reproduce from extra seeds that land above the water.
Tree - produces food above water, the food is more caloric than the bush food. which may fall into water and reproduces from extra seeds that land above the water.
Carnivorous Plant - Produces food, may occasionally eat a small fish, juvenile or potentially a dead fish.
Herbivores:
Anchorfish - Eats food, reproduces in numbers 2 and up, slowest breeding herbivore. Stays in the water.
Fishclown - Eats food, reproduces in numbers 2 and up, faster breeding than anchorfish. Stays in the water.
Clammoth - Eats food, reproduces in numbers 2 and up, faster breeding than fishclown. Can enter water but is faster at flying.
Hogfish - Eats food, reproduces in numbers 2 and up, faster breeding than Clammoth. Very slow movement and fodder to predators. likely to starve in competition with other herbivores due to their slow movement.
Eyepeller - Eats food, moves freely in the water, reproduces in numbers 2 and up, breeds faster than the hogfish.
Pufftacle - Eats food, moves freely in the water, reproduces in numbers 2 and up, breeds very fast. Faster than the eyepeller.
Pomanatee - Herbivorous, can only be eaten by the emperor fisher. Reproduces in numbers of 2 or more. When starved of normal food or plant seeds, pomanatees will rip aquatic plants out of the substrate and eat them. Very long lived, difficult to remove intentionally. Be careful adding a lot of these.
Nautilus - Slow breeding. Eats food, moves freely in the water. Immune to being consumed by the Gulpuff even as a juvenile due to the shell.
Predators:
Gulpuff - An all-female predatory fish that can eat juvenile fish and dead animals in the water. Can reproduce alone with only one in the tank. Reasonably likely to eat other members of their species if they are still juvenile.
Froggair - A semi-predatory species that can eat clammoths or normal food pellets or fruit and seeds. When eating clammoths they are much more likely to reproduce than when subsisting on plant material. Reproduces in numbers 2 and up.
Emperor Fisher - A flying predator. They can eat pretty much any animal. They will capture fish with their beard-like tendril and consume them periodically. Breeds in groups of 2 or more. Corpses from starved or aged-out emperor fishers can be eaten by gulpuffs.
Scuttlefish - The scuttlefish can only eat the Gulpuff. They are very long lived, and breed quite slowly. Can move freely in water.
Jetsnap - The jetsnap is one of the primary predators of clammoths along with the froggair. The jetsnap can only fly, but hunts clammoth. Reproduces in pairs, rather short lived and difficult to maintain.