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is it ok to leave the heater on all night? I heard somewhere I have to take it out when you reach the right tempp. it seems like if if i take it out it will go back to the old temp

get a thermometer and make sure it stays at the same temp

well im not worriued about that i have a thermometer. i just heard ur not supposed to leave it in all night

If you're talking about a heater for the water... set it and leave it in. The heater should have an internal thermometer that will keep the water at whatever temperature you set. Most thermometers have a variance of +/- 2 degrees F. Some of the better thermometers have even less variance (some of the very best and most expensive will keep your water temp within half a degree). This also assumes you have the heater adequately sized for the volume of water in your tank. Of course the best way to know where your thermometer is set is to have a separate thermometer. I'm partial to the digital thermometers. I have one of the little digital marina thermometers available at most any LFS for $10 USD.

Set your heater (or if you have a preset one)... and leave it in. It will turn on and off by itself. You are correct.... if you take it out your water might revert back to a cooler temperature than is desired for your turts.

Who told you to unplug it at night?I know everyone has an opinion, but i guess it's up to the person "asking" the advice to do a little more research before listening to first dummy that answers.
Just wanted to add that....lol
VeipaCray already said it all