Your last fish left alive is a corydorus -- cory for short; they do not like warm water. 88 degrees -- 31 Celsius is way too warm for it and it is not likely to last much longer.
I know many people use temperature to kill Ich but there are so many good chemicals on the market that will kill it if used properly without 'boiling' the water. Ich has different life stages and you can only kill it when it is in the active stage. Each active stage lasts about a week so you need to dose for three weeks -- the precise instructions will be in the box, follow them carefully.
The cory that you have will not do particulary well witht he substrate you have as they have barbels that will get worn away as it tries to get food from between the stones -- they prefer a sand substrate -- but it will survive, all other things considered.
My suggestion as you stand now is to reduce the temperature to 24 degrees Celsius, introduce a chemical Ich-killer and importantly, remove any carbon filtration as it will remove the medicine from the water. If your cory survives I suggest you continue with only that fish and over several weeks introduce other cory's one at a time and allow the filter to bring up a bacterial colony. Don't wash any of your filter material in ordinary water, just squeeze it out in old tank water -- if it even needs cleaning at all (and i doubt it will with only a very few fish (one) in the tank).
In summary:
Lower the temperature - do it now or the cory will die.
Chemical Ich treatment, properly used.
Regular water changes -- weekly and try to change 50% for the next few weeks, less later on if you like (I always change 50% per week and I have a 640 litre tank). Use Seachem Prime to condition the water as you put it in, it is efficient and economical.
Add fish gradually, do not try to hurry the bacterial colony, one cory every two weeks will be quite enough.
Eventually your bacteria will build and you can add fish more often and more at a time -- do not go silly, filter bacteria mutiply fast once established but they are not supermen.
One last thing. Try to find a dedicated fish shop; they know far more about the fish they carry and are more likely to care about those same fish. Large multiples have a fish section with the odd person who might have little more than a passing familiarity with fish-keeping. I'm sure there are some very concientious people in the stores but unless you know for sure, don't go near big pet stores. Ask around your area, I am sure there are reputable stores within fifty miles -- even in the States (it is pretty big after all).