hiya, i think i have the same problem ? it looks like a lump, but wen u get close u think its a cut, its fairly small, but worries the #### out of u rite ?
i posted on here abt it, and got told that i sud treat the tank with melafix. it cost me abt £5.99, but its fairly hard to find, depending on where u are in they uk.
if this is wot i have, pm me and i can send u the advice i got.