Mega enclosure (long term project)...

[Rushthezeppelin]

Ya proper sealing keeps it from offgassing. I plan on sealing with drylock when I get around to finishing that piece (I've got too many things on my plate ><). There are plenty of people who have been using it in the koi ponds for a long time. Just make sure it cures for a week and there is no gassing and it's completely waterproof. You can also cover foam in silicone and pat eco earth on it. You can do this with both sytrofoam and the great stuff brand spray foams.

[Simland]

Thanks for the info.