I wouldn't use those cork tiles - they are made of cork bark that is chipped up, and then reformed and glued together (something akin to the cork equivalent of particleboard). Unless you know what the adhesive is, and know it's safe, I would avoid it.
What you want to look for is 'virgin cork bark' - it comes from the first time they harvest cork bark of of a cork oak, and has the typical 'bark' look with a very rough, crevasse filled surface (I may be wrong here, someone correct me if so). Several places online sell it, sometimes you can find it in pet stores, especially if they have a lot of herps.
Another place to look would be places that sell orchids and epiphytic plants, as they use them to mount their plants on.
Finding a source online would probably be your cheapest option, but also you wouldn't really be able to see what you are getting.
Another thing I've used in the past is tree fern fiber slabs, works fairly well, not as pretty as corkbark when naked, but can support plants/mosses fairly well.
As to vinegar, that's more to test for carbonate on rocks, it's just an acid reacting and releasing CO2 (the fizzing). and vinegar by itself is iffy, it's better to use a strong acid. Also, all that will tell you is whether or not the rock is likely to affect pH and water hardness.
I returned the tiles because I didn't trust them. Thanks for the suggestions. I tried 5 different places in town and none had the cork bark. They said my best bet was getting it in from Brisbane. (1.5-2 hours away.) Somehow I don't see that happening. I can't find anywhere to purchase online but that might have something to do with my googling skills :/.
I managed to find some coco fiber and I've got some safe black silicone so I'm thinking of carving styrofoam background, covered in silicone then coco fiber. I am worried about the waterfall section though. If I have water running constantly over the coco fiber will it wear off quickly?
(I knew about the vinegar. Thanks)