reapplying diy rock background

Hey guys,
Im currently setting up a new fish tank using a diy rock background that used to be in my poison dart frog vivarium. the background is made of blocks of open celled foam hot glued together and covered in cement, painted with acrylic paint. this was running in nearly 100% humidity for about 3 years. unfortunately during hurricane Irene, i was displaced from my home and i could no longer care for my darts so i disassembled the tank but kept the background because it is beautiful in my eyes and i couldnt get rid of it. It is now going into my soon to be discus display tank and i am worried about this affecting water quality. will the hot glue used in the construction of this leech toxins into my water? if the hot glue is no good, i believe my best option is to seal the whole back of it with silicon, but i am not very partial to that. also, i am wondering what the best way to remount this to my aquarium glass. i would say silicon is the best bet, but there is a good amount of foam still attached to the concrete and is still buoyant so im not sure if silicon will adhere to the glass enough to counteract this. maybe countersinking a few stones into the back of the foam might help?
Thanks for all the help, i will be posting pics when i am nearer to discus move in day.

i was actually going through logistics and whatnot,, i think i will use this piece in a 29g for a planted community tank.. ive always been a fan of rams and have some experience with them so thats the direction im going to take with this.. the discus idea moved to a bare bottom 55 setup. although i wont be able to combine my piece+planted tank with the discus, i'm confident this approach will be better for the fish and myself as well.
concerning the leaching problem, ive been using vinegar and lots of rinsing with warm water for the past 2 days, would you still expect a drastic effect on water parameters?

what method of sealing would work in this application?

hey guys, i finally finished this project, unfortunately i did not take pics through the process. the concrete background is showing an increase in kh and gh, but with twice a week 50% water changes, it is slowly lowering. i estimate maybe another two months before it is completely inert. other than that it is doing great, plants are thriving and exploding with growth and the fish seem happy and healthy.
Fish:
-1 dwarf gourami(want to get him a female)
-5 cardinal tetras
-1 panda cory(will be getting more i just wanted to make sure he did well as the substrate is a little rough)
-5 otos
-1 rubber nose pleco
-15 glass shrimp
-assassin snail
-freshwater clam

Plant species include anubias, crypts, rotalas, and many more
Scape:
2 pieces of manzanita
concrete corner piece
1 rock from the jungle of puerto rico tested out to be completely inert

hardware
eheim 2213 180 gph with sponge prefilter
sphere airpump
aqueon 150 watt heater
coralife 65watt power compact fixture
coralife sattelite + LED fixture(beyond cool) (last pic shows my night setting at 10 seconds exposure)
DIY CO2 injection with film canister to increase contact with water for higher dissolved co2 in the water

separate 29g tank to treat and hold my water for WCs. airstone and heater so water doesnt go stale and to age it properly. i have a hose with an extra eheim quick disconnect to hook up straight to my filter to pump new water directly into the tank after old water has been siphoned out.