For the past few days I've noticed my fish, Ciel, become increasingly sluggish. Today he was pretty much at the bottom of his tank the whole day, barely moving unless I tapped on the glass (to make sure he was still capable of moving). This is what he looks like now. He is in a topfin 3.5 gallon tank which is filtered and has a heater to maintain the right temperature. His water was changed today after a month and I put in stress coat+ to condition the water. The only thing I could think of that caused this is that I hadn't thoroughly cleaned his tank until today. He's always been fine without water changes for a month or so (which I know should be done every week) but I think some food pellets that he didn't eat sank to the bottom under the pebbles and began to rot which gave him an infection? Today I completely rinsed out his tank, scrubbed off all the grime on the tank and the decor with a sponge, and meticulously made sure that every single pebble in his tank was completely clean. I then had to leave for nearly 5 hours and I come back to him still being at the bottom of his tank. What can I do? He's only 2 years old and he's always been a truly healthy fish with such a personality and would dance every time I gave him food. Now he doesn't even want to come to the top of his tank to eat his pellets, he's just laying there breathing and barely even moving his pectoral fins.