If this is a full grown Dojo Loach (over 6" perhaps) then I would use at least a 10 gallon tank as a hospital tank. A smaller Dojo could be hospitalized in a 5 gallon tank. Monitor the conditions very carefully, that would be a large fish in a small amount of water, and the ammonia level would rise very fast.
However, many medicines are dosed based on 10 gallons, so a 5 or 10 gallon tank is easiest to dose.
Bare bottom is easiest to clean. If you put the tank on a dark surface (or put a dark towel under the tank) the fish will feel safer.
Put a few stones in there, or an arch of driftwood. Something the Loach can hide under, but you can still monitor his condition.
Since you do not know what is causing the bump, it is hard to suggest a medication.
However, I would start with some type of antibiotic that enters the fish's system through the gills. Kanamycin is one such product. Some antibiotics also have some action against fungi. (This does not sound like a fungal problem).
If the bump is some kind of tumor then there is no cure.
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!