you may want to see about a mineral reducer chemical, I have no idea what high amounts of iron will do.

And I never answered your original questions:

1.how would that be attached to the back of the tank?
You need to cover the glass with silicon. I use GE silicone II. Let the silicone cure for 48 hours, then the GreatStuff (GS) will stick to it.

2.what is the best tool you have found to cut eggcrate?
Sharp blade. I have a mega sharp knife.

3.would mold become a problem behind the background? if so what do I do?
Mold won't be a problem.

4.whats the best way to cut the great stuff, with a heated knife ?
I use a utility knife. no need to heat, just give the GS 48 hours to dry fully.

5. can you paint great stuff? if so what kind of paint?
I have never painted GS. After the GS is dry, i coat with another layer of silicone II and press in fully dry coco-husk while the silicone is still wet.

6.once inside the tank can I add more great stuff against the glass, and it wouldn't stick?
See #1

__________________
My name is Liz
Do what's best for the animal, not what's best for you.

There are two very different materials that are called "eggcrate"
One is the pressed cardboard like material in egg cartons. The other is the plastic material used as a diffuser in flourescent light fixtures.
Which are you referring to?
I would think trying to cut the plastic "eggcrate" with a knife would be rather labor intensive.

__________________
Merlin,
What's Life Without A Little Magic!

i was referring to the third type of egg crate, the foam stuff used in bedding. :

__________________
My name is Liz
Do what's best for the animal, not what's best for you.

the eggcrate I was referring to is the light diffuser. check out that link I posted in I think it was the 2nd post, it really looks pretty cool.

For cutting the light diffuser your best bet is a hacksaw.
Unless you are very skilled with a good tablesaw.

__________________
Merlin,
What's Life Without A Little Magic!

Interesting. I never looked at that construction site. Never would have thogut about using the eggcrate. I guess if you want to be able to remove the background for some reason. I have my great-stuff attached to silicone, which is attached directly to the glass. I was still able to set containers for plants into it, i just had to set the tank on its back until the GS dried everything in place.

here is a good photo of how I did this.

__________________
My name is Liz
Do what's best for the animal, not what's best for you.

20 0z. soda bottles for pots. i like recycling, makes my wallet thicker. but yeah i'd like to be able to change the background if I get a different tank I could still put fish in it if I wanted.

I'm ALL about recycling. But I think that was a 2 liter

I can't see putting this tank to another use. More like different frogs living in it!

__________________
My name is Liz
Do what's best for the animal, not what's best for you.

I could be wrong, but Iv'e always been told once a tank's used for reptile's, it shouldnt be used for fish.

__________________
"Nobody can do Everything, but Everybody can do Something."

Why? Clean it with bleach, its good as new.

__________________
My name is Liz
Do what's best for the animal, not what's best for you.