gph

Hey guys, so I have a 10 gallon aquarium right now. It has two goldfish and two bronze corydoras in it (for keep algae off the substrate). I was thinking about upgrading the filter since it is for a 10 gallon, but goldfish are messy, and one of them is 6 in long. What would you suggest for...

So I got my protein skimmer today... turns out it is an Oceanic Biocube Skimmer.  Wasn't labeled as that though, but it was like 30 bucks so I took it.  Sadly no returns, and I'm not interested in buying another one.  I checked the air pump and it said CAP 21 gph, so basically I'm gonna get...