I would only clean the sponge filter media with tank water, just scoop out a good amount and swish and squeeze out the sponge in that. Some of the gunk in it is beneficial bacteria which actually cleans your water when it filters it. Killing all of the bacteria by washing it in chlorine tap water takes out the bio-filtration on your filter which makes the filter pretty much pointless.

Anyways, tetras are good beginner fish. Guppies etc. Selecting fish all depends on how much you are willing to pay up front and in the long run. Some fish require more than your regular flake food. I have 3 tanks, cichlids, puffer, and cherry barbs. Besides the cherry barbs, the cichlids and puffer are on a higher maintenance scale. They are fed mysis shrimp, brine shrimp, different kinds of algae, blood worms, flake food, and more. Whatever fish you like at your local pet store, research it online before you buy it. Then decide whether or not it is a good pet for your tank mates, budget, and time. This hobby is pretty much 50% research, 20% money and 30% maintenance time/upkeep.