Move the tank to a different area where it's warmer.
Heat the house a bit to reduce the stress on the heater.
Buy a higher wattage heater.
Set the heater on 88F and see if it gets the water closer to 86F. Do this when you are home and can monitor it for a few hours just in case it fries.
If that fails you can either use chemicals like copper or malachite green. Copper is safer unless you have shrimp in the tank. If you have shrimp do not use copper because it will kill them.
Malachite Green is carcinogenic (causes cancer) so be careful if you use that.
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The final option is to move the fish into a new container with clean water every day for 2 weeks. Basically you set up a couple of plastic storage containers with clean dechlorinated water in. You put a couple of plastic plants and add an airstone and heater (no filter).
You put the fish in this container for 24 hours and then move them into another container of clean water. You tip the first container out on the lawn and wash and bleach the container and everything that was in it. Then rinse and dry it out. You also need a couple of nets to move the fish with and the nets need cleaning after each use.
You move the fish into a clean container of water every 24 hours. The white spot parasites drop off the fish and sit on the bottom of the container. The fish lose some of the parasites each day and after 4 or 5 days, they are normally free of the parasites because all the whitespot cysts have fallen off them and been washed out of the containers.
When you use this method you have the heater set on about 26C (79F).
The main tank where the fish came out of is left to run and the parasites die off after a few days without a fish host.
