I'm in the process of creating a sponge filter for a new 10 gallon tank. My plan is to glue a drilled out 1/2" PVC
pipe to the bottom of a jam jar filled with broken up lava rock, a layer of polyester batting, and a sponge between the jar and the 90 degree elbow output nozzle. I made a spray bar to attach to the output as well, which I'll use depending on what tank inhabitants I end up getting. Airline with a bubble stone at the end will be fed through the pipe to the bottom of the jar. I ordered an air pump rated for 10g, but I'm thinking of getting something rated for up to 20g instead, just to be on the safe side.
My question is this: how do I know if my filter is effective enough for the size of my tank? I assume the power of the air pump is a major factor, but I'm guessing the size of the sponge (it's 2" x 4" x 4" that I plan on cutting in half and layering) is important as well. Anything else I should keep in mind? Is there a preferred method of testing the flow rate? If I diffuse the output, will this affect the filter's effectiveness? I'd prefer to have more filtration than I need, rather than just enough.