I'm thinking 30 - 40 gallons but that's a total guess, someone else would need to chime in there. It's ok to have bothe the UGF and a hang on filter, just be sure to be good and diligent when doing your weekly vacuuming as you don't want a mess growing in there. This can be lethal.... For this very reason, some folks use an Reverse flow kinda thing which I am not familiar with but I'm sure someone else can chime in. I'm thinking you should not get any additional fish for now, just get it going with those you have now. When you do get a handle on things, then we can think about if they need more and if your tank can handle it. One more thing, when you get the filters for the tank, they need to be running 24/7. And it's advised you get a heater too. The heater isn't to warm the water but to keep it stable. I had mine set for 74 and my fish did extremely well with it. I wish you the best of luck once this ball gets rolling for you.... be patient, you have about 6 weeks ahead of you.