I think he its saying to but another tank put the background in it get it cycled than switch your fish to the new 75 than break down your old one and sell it...that way your fish won't be living in rubber maid tubs while you work on your 75 now. Only way I see safely putting in a 3d background is to have an empty tank...
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Correct. No way you can do it with an established tank.
You also have NO idea how floaty the foam is. I siliconed the heck out of it and wedged it under the top frame.
I used a razor knife and just started cutting horizontal lines and prying chunks out. It is very messy but easier than I thought.
The paint I used is cement water proofing paint from Lowes call drylock.
http://www.ugl.com/drylokMasonry/masonryWaterproofer/latex.php
It is very thich paint and white. They tint it the color you want for free. IIRC I put on two or three heavy coats. I used only grey but should have gotten some dark greay too to add highlights. The shadows from the aquarium lights bo that already but it would look even better.
I let the paint dry a week and could have waited longer. Tank turned a little greay at first but multiple water changed cleared it up. No fish were harmed when tank was eventually cycled and intruduced.
And once again, dont build any permanent caves into it or you will be sorry later if you ever have to remove a fish.
Remove the top plastic rim from rge tank so the backround is all one piece and wedged in. A PITA but worth the extra effort.
Hope this helps....Bill