It depends on several things. What size tank(s)? What kind of fish? How many fish? What you are going to feed? How many times a day you feed? How often you change water? I use sponge,box filter(one that has floss in it) undergravel filter just to name a few. Any kind of filter will work,you have to understand how they work.

I use one that hangs INSIDE the tank. It has two filter mediums. One is charchol filled white woven mesh. The other is a cellious black mesh that would remind some of furnace air filter or a large mesh sponge.

Water level has to be kept below spill lip so hard water line level marks usually appear. Especially with my well water as limestone levels are high here.

Other problem is water flow is not adjustable. Because of this the surface and water current of tank is higher. I put mine in a 10 gallon tank (but it can do a larger one) and when I feed the flakes are knocked down and actually circulate the tank. The fish do not come to surface but try to catch food from the current.

I don't know about its sutability for fish or plants that like slower or still waters. Or for tiny fry. Except that when my friend was removing some of the weeds in her natural farm size pond I rescued some air filled sacks at edge and put them in my tank. She thought they were frog eggs which we were going to put back into pond.

To my surprise I returned from a weekend downstate to find I had fry in the tank. All that hatched lived. Once we discovered they appeared to be goldfish and not the bluegills they did not need to be returned to her pond.

They have grown and done well. I gave three of them to my grandchildren and kept two for myself since the 10 gallon is definately too small for 5 goldfish and one chineese algae eater and one other we have yet to identify. They are doing fine and taking on varied levels of goldfish coloring now.

Real plants will need to be secured, able to handle water movement. It may be best to plant them furthest from Filter. Plastic plants would be fine anywhere if weighted, secured.

Small corner sponge filters with air bubble tube are not as effective and definately release debri when lifted to clean out.