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 about adding another filter to the other side. What kind of filter would I need to benefit the first filter but not have too much flow?