Could be an infection, injury, tumor or a prolapsed rectum. Hard to tell without seeing it. First: do a partial water change, (40%) with a good quality declorinator. Then try fasting the fish for 24 hours and add Epsom Salt to the tank at the rate of 1 tablespoon per five gallons.
Watch for any changes in the bump over the 24 hours. Hopefully you'll see some reduction in size but if it continues to grow or if the area gets redder or covered with white or gray fuzzy growth, then you'll need to treat with an antibiotic.
Robin
Do a search for prolapsed rectum here on the forum. You should be able to find a picture or at least a good description of what it looks like. Might help you determine if this is what your fish has.
Watch for any changes in the bump over the 24 hours. Hopefully you'll see some reduction in size but if it continues to grow or if the area gets redder or covered with white or gray fuzzy growth, then you'll need to treat with an antibiotic.
Robin
Do a search for prolapsed rectum here on the forum. You should be able to find a picture or at least a good description of what it looks like. Might help you determine if this is what your fish has.
