Well, I don't use heaters in any of my tanks any more. I understand your concerns about too hot. So depending on your location, GBRs & cardinals may be a better fit.
I live near SF, CA. It gets very hot in summer but we use AC during the hot part of the day....& my tanks are larger. So I see much less temp. fluctuations than you might. If it's just during a short vacation when it gets hot, fish, even cool water 1s, will be fine. But if it stays that way all summer, you're apt to keep higher temp. tolerant fish more successfully long term
Nitrite is harmful to fish in any amount. If your strips show that you need to do frequent water changes (daily if you can) to get it to 0 ASAP. I've used strips, they're not terrible IMO. There are 6 in 1 strips that test for ammonia too, unlike 5 in 1. Neither is as accurate as API individual tests, but they a very quick to use. A cheapness hint, I used to cut the strips in half the long way (yes, I'm a frugal yankee, lol).