with 68-79 degrees F / 20-26 C. However they have their origins in the upper Amazon (as in Peru) and those waters are more likly 68-74 degrees F. Bloodfin tetras, the more robust Buenos Aires tetra and a number of other tetras either from the mountainous areas or pretty far south in South America, are comfortable in what we would call unheated aquaria.
Their cousins, the cardinal tetras are 1,000 miles down stream and do better at 10 degrees F warmer. Keeping Neons and Cardinals is not the best idea, because one or the other may suffer from "temperature discrimination". ;)
Common guppies will take those lower temps, but your 75 F/ 24C is a pretty good compromise and still tolerable, I would hope. for your fancy guppies. If your neons were young when you got them and your water has pretty modest mineral concentrations, you may even find them spawning in clean water, though no eggs are likely to survive in a community tank.
Neons which are raised in hard water are spoiled as spawners. The minerals will "gum up" their reproductive systems, a problem for many other tetras too.
Neons have quite a history in the hobby. Exported first to Germany, a few neon tetras were brought to the U.S. on board the German zepplin Hindenburg (obviously not that last fateful voyage). Aquarium Magazine editor William Innes (for whom they were named) got a look at and photographed them and a couple (I think) found their way to the Shedd Aquarium. They were quite a sensation.
All the best!
unc
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