DIY fish tank ornaments and background?

use styrofoam sheets and aquarium sealant or some kind of glue that's safe for fish. You can carve it out to the shapes rocks or whatever you want. Then paint it. Even if the ph rises you can use other things to bring it back down like driftwood or certain rocks. But besides that the styrofoam or sealant shouldn't affect the ph. I'm sure you could find stryofoam and paint in nearby hobby stores. Good luck