You really need to invest in some good filters, not to mention more water changes and filtration methods. It's not that hard, and that's coming from someone with around 24 huge tanks. Lol either way you will be happy with the results.
Do you plan on getting another few fish soon? If so what have you got your eye on? I personally like the eeltail catfish, and some of the rarer species. Something with a bit of a personality.