Heaters don't work well unless the water is circulating. I'm sure 75-78 temp is fine. You can try using bottled water in case you have something wrong with your tap water that the Stress Coat isn't removing - a foreign chemical or something. Also, what are you feeding him? In my experience with Bettas (and I've had several live over 4-5 years), Betta Bites have not been the best and some people tend to overfeed those. They cause stomach bloat and swelling. I found flakes to be the best, but some do well on the bites. You may have gotten some bad fish, too. With the new (er, old) betta craze, its awful the conditions they're kept in. I'm so glad you got yours a larger home. Do you have space above the water line where he comes up for air? They are labrynth (sp?) fish where they breath the same air as us. No direct sunlight. When choosing a fish, I make sure the color is nice and the fins are not clamped at all - a common problem in bettas. I'm sorry you're having this problem.

You should filter the aquarium he probably being poisoned by the ammonia.Either you get a filter or change the water every day by 50%.you could even get a sponge filter.

Use heater with water circulator(see through it that u dont over heat the water)& try using the leaves known as Indian almond leaves available in market or in any park[ALSO KNOWN AS Tropical Almond Leaves,Sweet Almond Leaves,Wild Almond Leaves,Sea Almond Leaves,Catappa Leaves,Java Almond Leaves,Ketapang Leaves or wild almond leaves in some places]comes from a tree known to produce a chemical that defends itself against insects and parasites. When the dried leaves falls into water, a strong brown dye is given off. The dye contains organic acids humic and tannins. The water which is darken to a yellowish brown (tea-like color) after a few days, is very similar to your Betta fishes' natural habitat and they will love you for it.
Humic and tannins from the Indian almond leaves lowers the pH of the water, absorbs harmful chemicals and The main reason is that their antibacterial properties minimize the chances of bacterial infections, improve health, stimulate the natural environment of fish who hail from soft, acidic waters, stimulate breeding conditions and harden scales which minimize skin diseases and pathogens resulting in bacterial infections that dwell on the epidermis of the fish which helps to create a soothing and calm environment for the fish. I use the leaves to help settle my Betta fishes when I first bring them home from the aquarium, which can be quite a traumatic journey for the fishes. Even Bettas can get 'sea sick' sometimes ... you will know it when their body turn pale and rest on the tank floor all the time.
The leaves can also be used to treat bacteria infection and help the Bettas recover from injuries. Some people have used it to increase breeding frequency. The first time I discovered the healing qualities of the leaves was when one of my Betta won't eat for a few days after I brought it home. Then a friend recommended I put a square inch of the leaves into the tank and let it soak overnight & dont remove the leaves for about 2-3weeks it will not harm ur betta. The next day, my Betta was sucking the blood worms like there is no tomorrow! at that time he about 7-8 months old & today he has completed 2yrs still fit & fine.....