Yes, the tank needs to be cleaned. On a 2.5 gallon the filter is more of a water mover than anything. The tank cannot hold a cycle, so the normal benefits of a filter on a larger tank don't apply. You will need to do one 50% and one 100% change a week. So two changes a week total. Tanks that are 5 gallons and larger can hold a cycle in the filter and only need partial changes once or twice a week depending on how many fish you have (after the cycle is established that is). Some people have managed to have planted tanks that need no water changes, but this is a hard balance to achieve, and there is some controversy surrounding this idea.