Here are some suggestions:
1. Raise your temp. to 85-88 F. The ich parasite cannot survive in its freeswimming stage at temps of and over 85F. the parasite can survive and reproduce at 82F, that is part of your problem.
The raised temp. also quickens the lifecycle of the parasite, speeding up the whole process.
2. I would do a 30% water change perhaps first and then raise the temperature.
3. I do not recommend the use of meth.blue to treate ich.. catfish like corys and other scaleless fish are quickly killed by it and much less stressful meds are available. I haven't used the product you mentioned but I use Aquarisol whenever I see ich and along with raised temperatures..this does the trick.
4. If you DO NOT HAVE corys or any other salt sensitive fish in the aquarioum
I would suggest using aquarium salt for FRESHWATER aquariums at the dosage suggested on the package. DO NOT use table salt or marine salt to treat a freshwater tropical aquarium, that way madness lies..
Guppies do quite well with the addition of salt at it aids the fish in its slime production and further nabs the parasites in its freeswimming state.
So,
Water change,
Slowly raise temp to 85-88
Dose with aquarisol
Use freshwater aquarium salt [if you don't have salt sensitive fish]
And you should be on your way to recovery..
GOOD LUCK!!