I got my first Betta in July and he had been doing fine until we got our first good cold snap here. I didn't realize how cold his tank had gotten until I came home and he was on the gravel gasping for air. Thankfully Walmart was still open, and I managed to get him a heater and thermometer that night (2 days ago). He moved up towards the top of his tank and hid behind the filter gasping instead of sitting on the bottom. I woke up the next morning and went to check on him and he was perfectly still, leaning on a plant. I figured he was dead and went to scoop him out and he dove into the gravel so hard I heard him hit the plastic. He stayed like that for most of yesterday. Whenever you got too close to his tank, he'd either wedge himself behind the heater and get stuck or dive into the gravel.
He's looking better today, but he's still sitting behind his filter and every once in a while I see him sitting perfectly still, no gill or fin movement. He hasn't eaten since Wednesday morning. He usually gets a few bloodworms in the morning and at night, and every 7-10 days, I don't feed him for a day. Until Wednesday, he used to come screaming up as soon as he realized I had food. He's got a 1.5 gallon tank with a new filter, small heater, the temp stays around 77 degrees, and I put about a 1/2 teaspoon of aquarium salt in yesterday after a 50% water change on Wednesday when he first started acting sick. Does it sound like he's recovering or is there anything else I can do for him? How long can he go without food before I should be worried about him? Also, it looks like he may have ripped his swimming fin up by his gills when he was diving into things, is there anything I can do to help that repair itself?