New betta is sick

[Aqua13]

I got my first betta a little over a year ago. Last week I decided to get two more. They all have their own 1 gallon tank with a heater and plastic plant. The water temp is 76. I do a 50% water change 2 times a week. I feed them a diet of pellets and blood worms. Two of my fish are doing great but one new guy isn't looking so good.

He looked really healthy when I bought him. About two days later he began to clamp his fins. Then his color started to fade. When I got him he was a dark red with a hint of purple. Now he is a light orange/ yellow color. He spends most of the time in the corner at the top of the tank. Sometimes he will lay at the bottom. The past couple days I noticed him having some trouble swimming. He moves very slow and doesn't seem to have control of his body (not swimming in a straight line). The best way I can describe it is he looks kinda like hes drunk. He will also start to sink to the bottom of the tank for a few seconds and then start swimming to the top.

He also hasn't been interested in food for the past few days.

Could he possibly have SBD? Any ideas how I can get him better?