Ok. Well first thing you need to get your water clean, so daily 50% water changes until your ammonia + nitrite combined are no higher than 0.5ppm.
That product contains malachite green which can kill off your beneficial bacteria and crash your cycle.
The thing with ich medications is that it only kills off ich in its free swimming stage. It says that on the instructions. After infecting your fish, it feeds on the fish, this is when you see the classic signs. Then it drops off and finds its way into the substrate for some time to reproduce, and then goes into its free swimming stage at which point your treatment can kill it off. The free swimming stage can be several weeks after your infection clears up.
Whats happened to you is you treated while the fish was infected at which point the medication isnt doing anything. The feeding stage lasts a week or so and your fish either survives this or it doesnt. Higher water temperature speeds up the ich parasites life cycle, shortening the infection stage, giving your fish a better chance of survival. You then carry on dosing medication for a few days, but again your medication isn't likely to affect ich at this time either because it could be weeks beforeits free swimming again. You then stop dosing medication because you think the ich has gone, but its not, its going through its life cycle waiting to infect a fish with a weakened immune system. Which happened quite quickly because you lost your cycle.
First thing first, you need to recycle the tank. Also, raise the tank temperature to 30c (im assuming you are uk based and not using Fahrenheit) to speed up the ich life cycle and get it out of its system. Do you have a quarantine tank? Much better to heat treat in a QT so as not to stress the other fish.