Your tank is already on 80F so just turn the heater up to 86F and let it go up overnight.
Do a big water change and gravel clean the substrate before increasing the temperature.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
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Keep the temperature at 86F for 2 weeks, or at least one week after all the spots have gone. The heat will kill the parasites and you won't have to use chemicals.
The pleco will be fine at 86F for a couple of weeks.
If the heater can't keep the water warm enough, get some 1-2 inch thick polystyrene foam sheets and tape them to the back and sides of the tank. Have a coverglass on the tank to help trap heat.