78 in the winter...summers they warm up to 82-84 with no problems. I try to keep fish that won't mind warmer water because of the hot summers, like oscars, texas cichlids, etc.
I like to keep temps around 78-82F. I set heaters at 78F and warmer for thin bodied fish like angels or very young fish because they can't withstand the cold.
depends of what kind of fish... if discus you need high temp... goldfish... they can go as low as 70. but a range would be 76-80. research on your fish's preference it would give you a concrete idea. but before anything else... get a reliable temp gauge (i'd go digital).
sorry i have to dissagree on the digital temprature gauges I had 3 that iI bought all at the same time all the same product all gave false readings they were the best marineland had $49.99 a peice guess where there are now? I use the lcd stick on the glass now , and have a automotive temp probe that i use to check the accuracy of the readings
i only use stick on the glass and it seems to be amazingly accurate, it's always what i set the visitherm deluxe heater to, a lot of people don't think they're reliable because they're on the outside but in my experience they are completely accurate
I keep my tanks about 78f.I like the stick on thermometers as oscars try to swallow the floaters.I do have 4 floters I move around to verify all therms reading the same