Good advice above, some additional thoughts. Water changes are always good in moderation, a 10-25 % water change every week is only going to promote a heathier tank, as when adding water back, you will airate the water some. The smaller the tank the more impact the nitrogen cycle will have on your tank.
http://faq.thekrib.com/begin-cycling.html
Water changes can reduce the affects of the Nitrogen cycle, and good bacteria in the gravel can help reduce the affects also. However the bigger the tank the smoother the cycle becomes. I would recomend a bigger tank, or easier, bringing in water samples occasionally to your local pet store