goldfish and koi are harty fish, if you aclimate them to the warmer water they will be fine just do it slowly. The koi and fancy fish in my pond outside did fine this summer when the temp spiked and the water rose up to 80. If your that worried about the temp just drop it slightly under 80 and add a bubbler to stir up the water. As far as different types of loaches i have four YOYO loaches that go crazy over snails. I actualy keep snails breeding in their own little tank to feed my loaches and my puffer. I dont think you could raise the temp of the tank high enough to kill the snails without also killing your fish.
