Please help/Betta

Please, i need help, i have a male fish ive had for about 8 months now from my biology teacher whom she got from the previous teacher, i went on a week long vacation and had given him a 7 day feeding tablet. I believe he ate it all and became lathargic after and when i got back home he was able to swim to the top but would back to the bottom. He was not a terribly great looking fish before as he had fin rot from his previous owner but i was able to stop it with a treatment of Erythromycin and General cure just to be on the safe side. I attempted feeding him the pea trick but he didnt even notice it. If anyone has any suggestions of what i could do or can do please help.
My parameter are:
7.6 ph
0 Ammonia
0 Nitrite
40 Nitrate
Tank: 3 gallon, plastic halfmoon with a single small plastic plant with small and medium size rock substrate.

In the picture, it looks like his scales are sticking out. If that is so, then he has dropsy, which is when the swim bladder fill with fluid. It usually comes from some sort of bacterial infection. The only possible way to cure him would be to find out what is actually causing the dropsy. Unfortunately, this is almost impossible. You could get some medicines and see if they help, but at this point, the best thing to do is euthanize him.

(Also, I noticed that your nitrates are 40ppm. Nitrates should never go above 20ppm.)

My guess as to what happened with your fish is that, when you left and put the food tablet in the tank, it released a lot of extra waste into the water which then made the water bad for the fish, causing some sort of illness. That is where the dropsy came from.