CONGRATS!! on a 110 gallon tank (*tank envy*). Your turtles are very pleased with you and would tell you if they could.
Is a stock tank made out of plastic? If it is use ties on the top and you could keep the bottom from moving with two rocks or anything that can "wedge" it so the turtles can't move it.
Two heaters on either side would keep it on the bottom.
Post a pic if you can and I will see if there is anything else I think of.
I'll see if I can take a pic of mine to show you what I did.
Here are some pics.
I blocked one side with the heater and the other direction is hooked/ wedged into the sunction cup.
I have two female red ear sliders.
Chimpy - 5.5 inches
Chompy- 6.0 inches
Both were born about 7/2007
Got them -- 9/2007