few months it is going to be 8 inches long, which is way too big for a 5 gallon tank, and will start sucking the slime off the fins of the other fish and kill them. If that is what you have, find a new home for it. Same thing if it is a Pleco, except it probably won't hurt the other fish. If you got an Otocinclus or Oto, then it will stay small and you should keep it. You can tell the difference by where it is swimming. If it is always on the bottom sifting through the gravel, you have a fish the won't work in a 5 gallon tank. If it cleaning the glass on the sidewalls of the tank, then it is an Oto. I like Otos. If you do want a bottom dweller to help keep things clean, add one of the Corydora catfish. They usually have a couple of different kinds at Petsmart. They will stay small and are much cooler than the other sucker fish.

It does sound like you are feeding them too much, which is causing the water issues. You shouldn't have to change the filter more than once a month. I wouldn't add any more fish until your tank has completely gone through the Nitrogen cycle. And even then, 3 or 4 small fish is the max for a 5 gallon tank.