Hi Soyk,
I'm sorry to hear about your betta. In the betta's favor, though, is the tank size and temperature. I always recommend that a betta be kept in at least 3-5 gallons, with a filter and heater. So the fact that your betta has good tank conditions is a plus!
Do you know your water parameters -- ammonia, nitrite, nitrate? Such a large water change (70%) may end up stressing the betta, or any fish. The replacement water should match as closely as possible the existing water temperature and pH. When you replace such a large volume of water, it's sometimes hard to match the parameters of what the fish was used to.
I remember when my previous bettas acted this way -- just laid at the bottom and didn't move or eat. Don't give up -- although one of my bettas died like this, the other one recovered and lived for about 5 more months!
To help your betta get to the water surface, lower the water level. Bettas do use their gills, but they have a special organ, called a labyrinth, that can utilize oxygen from the air. Bettas need to get to the surface to fill the labyrinth with air. Remove about two gallons of water. If you have a heater that is submersible (this is ONLY if it's submersible!) put it tilted or horizontally so it's still covered by water up to the minimum water level. Keep an eye on the temperature in this smaller volume of water.
An antibiotic can't hurt. I have had success with BettaMax, or the med that pulled my betta through once was Jungle Fungus Eliminator. One capsule of BettaMax can be used in a gallon of water, but read the directions on the Fungus Eliminator to give the correct dose for a small volume of water. Good luck!
