Making sure waterfall/pond is safe for reptiles to drink from.

[kriwu]

The silicone sealant I have is for aquariums. I am just asking, are you suggesting I "coat" the rock putty with the silicone sealant? Is that going to make it safe as a barrier to block out chemicals etc?

Yes, you would want to smear a pretty thick layer over the putty and let dry for 48 hours before putting any water in it.

What about plants? Can I use any plants to test the safety?

Probably not a great idea, as plants will react different to chemicals than animals. The best option might be to get a water testing kit, one for more than just ammonia/nitrite/nitrates.