I use Aqua-Safe. I put it in as I fill the tank after water changes. I've never noticed my RES or map turtles having any discomfort. They don't seem to like the hose being in there though 
Another piece of advice: your tank looks nice and clean, but put more water in it. You're wasting about 1/3 of the tank which not only means lower water quality than if you had more water, but less room for the turtles to play. These turtles love deep water. My water is about 12" deep and I'd have more in there if I could keep them from climbing out from the basking area.
Your RES has some nice color.