DIY Background for dart frogs

Im currently redoing my dart frog set up and i have a tank i want to make a foam background with. I would like to make it look like stone and ive looked online and people in the fish community have commonly used styrofoam and concrete. The only problem they have is concrete can increase the PH level, so they will cure it and leech it until it is safe (?) my question is will this be just as safe (?) for dart frogs? Im currently trying out BONDO body filler over the foam to see if it dries and is safe, im going to take the dried piece and then place it in a tub with feeder fish to see if it affects them in anyway. Any experience or thoughts on this is greatly appreciated!