As for his water - it is good to have an idea of the readings - ammonia and nitrite burn their skin and need to be kept at 0.00 at all times - ammonia is produced in their waste and left over food particles. (you reduce the values by doing water changes).

Filtration with a mature filter (google the Nitrogen cycle and cycling a filter) help keep the levels under control in between water changes for you.

I have a feeling this growth may be a tumor - for which it may be something he lives his whole life with. In that case the info regarding his system will come in handy keeping a happy, healthy little guy for a long time coming.