Cement use in FOWLR tanks

[Delphinus]

I'm in the process of locating some Thorite cement. No pH spike, can be used in the tank right away. Theoretically, any polymer based patching mortar ought to be the same situation, but Thorite has a proven history to be safe in aquaria.

Regular (Portland) cement is OK, but you have to cure it properly, for example keep the rock in a toilet tank (if small enough) for 6 to 8 weeks.

Bear in mind when I say "it's OK" you still have to be careful. Have heard a few stories with people having their chemistry thrown off for upwards of a year or more because they did something off with their DIY rock. In a FOWLR it may be less of a concern but even fish can be stressed out by wacky params so it's still important to proceed with caution.