Merlin is lethargic. Mostly he lies on his side gasping for breath, and I have no idea what's wrong with him.

He doesn't look at all different; he eats and comes up for air (and he always comes to say hi--the little sweetheart

). But mostly he hides.
The tank is 3 gallons. It is not cycled. Heater keeps it at 78 degrees F. I do a 70% water change every other day. I know I should do 100%, and I used to, but even though I tried to catch him in the net as quickly I could, he kept getting away from me. It was very stressful for him and he would lie at the bottom for a very long time after I was finished. He scared me enough that I stopped changing 100%.
I don't have a test kit for the water. I've never bought one before and I don't know what to look for.
I don't have another tank that he could go in ATM. I don't even know if that's the problem. What could be wrong?
Help please?