OK, I'll be the first to comment on no water changes...
Especially on as small of a tank such as a 5 gallon water changes are important. The smaller the tank the more important they are. Even if just changing out a gallon or two in a 5 gallon will probably result in fish being more active and better plants. I'm not as big on water changes in either amount or frequency as many here but, especially on such a small tank, it will likely lengthen the life span of your fish.
The thing is that in nature water is constantly being changed out whether due to the flow of a river or the inlet and outlet of a lake. Water changes are to try to duplicate this by exchanging old with new as is done in nature.
Also, if you use a siphon to draw out the old water, you can vacuum the substrate to remove old rotting food and such. Your small tank will just be healthier with such water changes.