How many gallon or litres is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
The lump does it look hard of soft like filled with fluid.
Does the lump look like a cauliflower.
Old male Geophagus will often develop a lump on their forehead (like male Frontosa). It is a build up of fatty tissue that is used to show the females that this male has a good territory with lots of food. It also acts as a food reserve during lean times.
If it is red or white in colour then it is something else and a picture would help to diagnose the problem.
sorry guys been away for a bit. I will nip to the shops today to buy batteries for my camera and put a pic up for you. The lump does look like a fatty lump as colin predicted