Making caves for aquarium

[Lauri]

I've been thinking of making some caves for one of my aquariums, I've heard polymer clay and normal clay are great for this kind of project. Anyone know anything about it? I don't know how that works, but wouldn't they clay crack if it's under water for too long?

[Chase]

I would recommend using cement for this kind of project, cement is used to make live rocks for reef tanks. If you're going to use cement for your project, you have to take into consideration the PH levels, it will skyrocket for a few months, but after that it will be safe for your fish.

[Chix]

I would worry about cracking with polymer clay, but I would be concerned with anything that might leache chemicals. Cement and Polymer clay are fine, use what you think is best and easy for you.

[Noffsinger]

How long does the cement have to sit in the water for the PH levels to go down to normal exactly? Is there a general rule or do you just measure every once in a while?