Hey, I just bought a betta fish about a week ago, i put it in a 2.5 gallon tank with filter and heater and after about three days it started laying at the bottom of the tank and only coming up for air (its fins are also a bit clamped). I checked the?

Bettas don't need a heater and they don't really need a filter but thats up to you but the water may be to warm and he may be in shock. I have 3 betta fish and they are all in their own cilinder glass "vases" and I have had one for 2 years and the others for about 8 months.

It may be sick. Does it look like it has ich or velvet or anything? What is the water temperature? The person who has there betta in a vase your betta is just surviving its not thriving. Did you aclimate the fish? If so how? And bettas need HEATERS and FILTERS. IT IS ABSOLUTELY NECESSARY.