if you haven't liquid water tested the tank for ammonia, nitrie or nitrate...these three chemicals could be the culprit or one of each

ammonia could be very high...you should also test the tap water for all 3 chemicals

I agree also, stop using ph buffering agents...fish, especially the ones you have, can live pretty well with a ph of 8.0-8.2

extreme swings in a tank can kill your fish quicker than you may think especially with temperatures swing and ph swings

your tank appears to be about 20-30 gallon tank (non-metric)...150 watt heater will help; purchase at least 300 gallons per hour (gph) filter preferably with bio-wheels (retains good bacteria better in order to prevent problems with ammonia)

it is good you have raised the temp to 82, then to 86 for 2 weeks, this will help slow down any parasite that may be causing the problem...be sure to add more surface agitation for more oxygen while you are doing this

once you get the ammonia, nitrite and nitrate liquid test kits, let us know the readings

I keep my quarantine/hospital tank running all the time. Keeps it cycled and it's basically the same coming out of w/e tank I pull them from. I generally test the water before I add the fish. I always add stresscoat before adding the fish as well. Half dose. I keep raggety plants in there too to help the ecosystem and grow up some.

API Master is a liquid test kit and is more accurate than strip tests.

Good gawd your tank was severly overstocked, and still is by my thinking. The tank is a 20gal

2 bettas (male and female) (not recommended to keep with guppies)
2 corydoras (require a school of at least 4)
1 guppy + 5 baby guppies
2 swordtails
2 white skirt tetras (require a school of 5-6)
2 head and tailight tetras (require a school of 5-6)
1 platy
1 molly
3 neon tetras (require a school of 5-6)
1 lemon tetra (require a school of 5-6)
2 serpae tetras (require a school of 5-6)

+ what you lost

1 black ghost knife fish (these grow huge!)
1 molly
2 guppies
2 sepae tetras
4 neons.

Considering the filtration you have for that sized tank and the way too many fish in it. I'll bet its either an ammonia, nitrite or nitrate problem.

Raising the temperature in a tank with these issues will make it more toxic, be aware of that :)

And btw, keeping a male and a female betta together is asking for trouble, especially in such a relatively small tank. He will almost certainly kill her eventually, and probably your guppies too.

If the liquid kit seems too overwhelming or too expensive, just get the strips. Mardel is another good brand. I use both, and i think the strips are fine for most intents and purposes. Not trying to steer you away from the liquid kit, but you need to test your water immediately, and the strips are good enough to tell you if there is a problem or not.

Fyi, i wouldnt get another Ghost Knife Fish for your 20 gallon tank. I've had mine for about 3 years now and its seriously pushing 13-15" long. They grow far too large for the other tankmates you have. I think my 47 Gallon is too small for him sometimes. I see baby ghost fish at the store and can't believe that mine used to be that size.