This MR is a nice place,,,,mostly.
So far the advice people are giving you is good. I had fresh water tanks for 20+ years,, and only recently switched over to salt.
Fresh water fish are much more forgiving than marine fish. Your fish should be fine at this point.
Just make sure you change some water every week.
If it were my 30 gallon tank. I would try to change 3-6 gallons a week.
Be sure to buy a gravel Vacum. A python hooks right up to a sink & makes thing very easy.
Suck it out. Flip a valve & fill it up again. it is probably the best $40 investment you can make for a fresh water tank.
Also adding some stress coat from time to time will help keep your fishies scales & fins in healthy shape.
Believe it or not fresh water fish need salt also...NEED? Well I guess not need, but Prefer.
You can buy a little box of "Aquarium salt" at any pet store. this will help keep the fishes immune system strong. Do Not use Marine Salt for salt water tanks. You could even use kosher salt. But the first time. Buy the little box. It has good instructions on it & I forget the dosage right now.
If you have more questions. KEEP POSTING. Nice people will give you answers.
Also change you filter media once a month. If you fail to do this. Nitrates will build up in your tank from all the rotting crap in there & when you have some time and a few spare dollars. I reccomend that you swap out that filter for a Aqua Clear. It is by far the best hang on tank filter you can buy and easily maintain.
http://www.aquariumguys.com/aquaclear.html
Also: set up an Account on Cichlid Forums, or any fresh water aquarium forum. Just cruise through beginner posts and you will learn alot.