On which sub-site should I ask questions on aquaterrarium construction?
Is there a pet-"overflow" that allows question about terrarium/aquarium construction and not just pet handling?
Is the DIY sub site suitable otherwise ?
There is a Pets Stack Exchange site that is about to go into beta phase. I think it would suit your needs.
The Pets beta hasn't started yet, but you can commit to the site (click on the Commit!) button in the link, and you will get a private invitation as soon as the beta begins.
Note: Your question would be off topic on DIY site.