not enough oxygen

When my fish would die in the past, I would always find them in the filter intake. Now I haven't had this problem but I'm afraid my fish don't have enough oxygen. Because it's a 10-gallon filter in a 20-gallon tank. I can't replace the filter because that's where most BB is stored. I'm thinking...

I have a bubble wall in my aquarium and I was wondering how to get MORE oxygen in my tank, other than live plants. Should I put in another bubbler, like an air-stone and have them run at the same time? My little cories just don't seem to be getting enough oxygen. I also keep my filter running...