When is the last time you fed the little guy? Can you show a pic of where the lump is? How far does if move when you massage it? When i first got into garters i was using tilapia as a staple because i had read that it was a safe fish and i found out that if you give them big pieces of tilapia or let them eat too much at one time that it's hard to digest and can get hard like a stone. This may or may not be the case in this lump that is moving around but just thought i would share. I thought if they could eat something the size of a minnow that i would cut the tilapia the same size but it's just too much meat for them to digest sometimes so it gets hard like a lump or stone. If you think about it the whole fish are hollow with bones and organs and probably digest faster. You also have to be careful if you buy frozen tilapia fillets because of what it could be treated with.