If the lump appeared overnight, then it's not a tumour and is most likely a wound as mentioned by Andy.
To stop the fish fighting you need more of them. Tetras need to be kept in groups of at least 10 of their own kind and more is better, especially with fin nippers like black widow/ skirt tetras. These fish naturally live in groups consisting of hundreds or even thousands of individuals so even 10 in a tank is a small group for them. When they are on their own or in small groups, they stress out and start biting things. However, don't get any new fish until after this has been dealt with.