Ich can take a few weeks to eradicate, because you are trying to kill new hatches of the parasite. When ich is on the fish, it is nearly immune to treatment (protected by the fishes slime coat) only the young trophonts are susceptible to treatment. Raising the temp makes them hatch faster, so the chemical does its job.
As to temp, there are different varieties of rainbows, those that come from northern Australia would be more heat tolerant, those from southern Australia prefer a cooler environment.
Angelfish also prefer slightly lower temps than discus.
The idea that all tropical (?) fish live in the same temp is a fallacy.
And optimum, and tolerate, are very different things, some fish live in a wide variety of temps, so optimum is not needed.
Some fish are much more sensitive, and have a smaller window and require optimum temps to thrive.
I have different tanks with different temps for different species.
I killed about 20 haitiensus at normal tropocal temps (mid 70s) until I realized they needed mid to high 80s to thrive.
And had very sickly Gymnogeophagus until I learned to forgo heaters in the their tanks.