A lump on the side of a fish can be an indication of a health issue. It could mean that the fish is suffering from an internal infection, such as dropsy or kidney disease, or it could be caused by an external injury due to another fish in the tank attacking it. In either case, if you see a lump on your fish's side, it is important to take action quickly and contact your veterinarian for advice. You should also monitor water quality and pay extra attention to the affected area so that any changes you notice can be evaluated by your vet right away.