Just got a 30 gallon fish tank for a bargain last Tuesday and now I've got 5 platies, 4 cories, a few large snails, a pleco and a molly. I'm planning on adding a few tetras of some kind in a few days too. Very happy with my new tank!
Did you cycle it? I'm guessing you didn't since the tank was recently set up and it takes about 4-6 weeks for it to cycle. So you'll want a reliable test kit and you'll want to test the water everyday to make sure the water doesn't become dangerously toxic. If ammonia and nitrites are present you can do a water change to decrease the levels present.