Hey just like they said a thermostat is your best bet with 5 tubs.A rheostat is just a more expensive dimmer switch you find in your home and there not accurate at all especially if the temps change in the room i have a dimmer switch myself but i only have one tub right now and it fluctuates so much sometimes a 10 degree difference up or down, i ALWAYS have to check it like 10 times a day and with a thermostat you just have to pretty much set it and forget it especially if you get a good one.I just did the dimmer for something quick and easy and cheap at first but i am definitely getting a thermostat im probably getting the ve-300 but because you have 5 tubs and assuming there all filled stay away from rheostats as much as you can there just more of a headache than anything else thermostat is the way to go yeah there more expensive but to keep your reptiles save and there less work for yourself.Hope this helped you.