You definitely need a thermometer, so u can make sure his temp is stable, it needs to be btwn 76-86* F. I keep mine at 81-82... The most important thing, is that its stable, and not below 76* F
Also, It depends on what your indoor temps are (the temp of the room his tank is in) if the room is 80 degrees, then his tank could be around 10 degrees lower, (which is 70* F - which is not warm enough) but it depends on tank size, so you *need* a thermometer, the glass kind that (usually have a suction cup) go inside the tank r best... They only cost around $2... But, the kind that stick to the outside of the tank are *not* reliable enough
Alot of times, people that have warm temps year-round like what you are talking about, will have a good heater (the adjustable kind) they set to keep the tank's temp from getting below a certain temperature.