Terrariums

Hello all.

I'm a long time hobbyist fountain maker and an off and on keeper of reptiles and amphibians since I was a wee lad.

I'm embarking on an ambitious project to build a multi environment paludarium, connected by a running fountain (my goal is to create a desert environment with a small spring where the spring then runs into two part swamp environment and finally a purely aquatic environment.

I DO plan to keep different animals in the different environments (I even plan to make a river bank burrow for my hamster)... but before everyone crucifies me, I'm planning to make sure the animals remain isolated from (and largely unaware of) one another, except for the running water, and YES I know MANY amphibians are poisonous to other animals. I'm going to do all my homework to make sure I'm a good small critter daddy. I'll post videos and pics of the build as I go through it (I'm still in the materials assembling phase) and I'll be open to animal safety feedback as I do.

My question is about safe materials to use for making the water ways. I don't want to use something that leaches toxins into the water and harms my critters.

I have some Instant Ocean HoldFast Epoxy, which I thought will be good for gluing rocks and stuff in place (and which is "fish safe"), but I really want something like a grout, cement or modelling clay. There's stuff I've used for fountain making in the past, but it usually has chemical epoxies and resins in it that I don't think I want to expose the animals to.

Any ideas?