Oh! I have a question about that too..
I have both a powerhead under gravel filter and an on the back filter..
The powerhead is rated at 175gph
The filter is rated at 100gph
My tank is only 20 gallons so 100gph is what's best right?
I only have one Cory cat in it right now
The powerhead makes a very strong current and the Cory swims up against it and I don't understand why.. When I put just the filter on though he just lays on the bottom..
Also occasionally he goes up to the top like he's getting a breath of air?
Thanks for the help!