Most species can take higher temperatures than what they prefer for quite a while, and the Phoxinus spp. are no different. The most important thing to remember, as sandtiger has pointed out, is to increase circulation in the tank. It is rarely the heat that gets fish, but the diminished Oxygen in the tank due to the warmer water. If you through some airstones in, this should handle the Oxygen issue. I would raise the temp at least into the higher 70's for a few days. Keep an eye on the fish. If they appear to be unduly stressed move the temp down a bit. Formalin also works extremely well for ich and can be used in conjunction with the higher temps. There's a formulation tht includes methylene blue that is my favorite. Good luck.
Oh yeah, what exactly is spawning rash??