If you are going to stick with a 5 gallon tank I strongly recommend you heavily plant it. That will help keep the water quality higher in addition to your frequent water changes. If you just can't do plants, just keep the water clean and you should do fine.
Maybe about half the gravel you currently have too. No need for more than an inch or so of sub if it isn't growing plants.
How old is he? If he is less than 6 months old there is a good chance he will outgrow this tank. If he gets to be 4 inches of body (from mouth to tail base) you will probably have to upgrade him.