These are among the hardiest fish known.

It doesn't matter. As long as it's above freezing and below 120F or so, they're happy. They've been reported in the wild at 100 degrees Farenheit in Madgascar (Loiseelle) and showed absolutely no sign of distress.
Water Chemistry Madagascar species are a bit more fussy than P. playfairi from the Seychelles, which can tolerate full seawater. Madagascar species are strictly freshwater fish.