So, I just got a 20 gallon tank today and I'm wondering how much surface area I need for a diy sponge filter. Before I measured it, I thought the tank was 10 gallons (I got it for free off Freecycle, and the guy that gave it to me thought it was 10), so I made a sponge filter for about that size.
I have a sponge filter that's about 2.5 inch on each side, and it's buried under a light layer of gravel to weigh it down. I have it hooked up to an air pump that's rated for 20 gallons, and no air stone (the tubing I'm using for the lift tube is just a little too skinny for the air stone to fit in). The gravel layer is about an inch and a half thick, and it's moderately planted and has several PVC pieces for hiding. I have my betta (Franklin) and 6 cories in the tank for now.
I'm starting the process of cycling the tank, and I know having the gravel and filter is absolutely crucial. My question is, what size sponge should I have for the filter? And is it ok to have it buried under gravel?
Also, on a side note, Franklin got hurt when I transferred him to the cup to switch tanks- part of his fin got stuck in between the tank wall and the cup and some came off. He's acting fine, but do you think I should switch him back into his little tank (1 gallon) for a while to make sure his fin heals ok?