A sponge filter is for biological not really mechanical. I like Fluval c series hang on back. They actually have mini wet dry part thats really good for bacteria. You can just fill the rest with pillow stuffing. The waterfall effect can be cured by just building up the bottom with egg crate and rocks.

I imagine with a frog like this you would probably have to change 50% of your water 2x a week to keep nitrates at a low level.

You need a legit water test kit. Make sure your levels stay optimal.