I know this must be a question posed a hundred times but every care sheet I look up gives me different answers.

My tank is unheated and currently 78. It's going to get much warmer in the coming months and I'm a bit nervous about the water getting too warm. It's a 5.5 gallon.

How warm is too warm? Also, the tank doesn't feel that warm at all so I'm curious if the mercury thermometer is wrong. Mercury is supposed to be pretty good from what I understand. Thanks for any help!