My guppy had been acting lethargic for a bit so I put him in a 2.5 gallon hospital tank where he's been separated for a couple weeks. Yesterday I found a raised bump on his side, maybe half a centimeter from the base of his tail. It doesn't appear to be filled with fluid, and it isn't discolored. I couldn't find any answers online, so I simply added a little extra aquarium salt to see if that would help. Today, though he is still eating and does not have clamped fins, he is even more lethargic, laying motionless on top of the heater and sometimes trembling. I've done one water change in case I accidentally put too much salt, and plan to do another. Unfortunately I don't know the water parameters since test kits are a bit expensive, but if it comes down to it I could buy one.