Yup my first Engineer goby's The tank is a 100 gallon QT tank till my 180 is brought online in 3 months. Regular river rock will not hurt anything. Been setup for about 6 months this way. Not to mention it's the natural rock in our Oceans here. Doesnt leach anything off.
Please tell me about your tunnel system. I was wanting to do the very same thing with mine, or atleast I had that idea, but haven't been able to find out if anyone else has done this. I want to set up a entire PVC tunnel system under the sand in the new 180 gallon, and put egg crate on top of it then about 2 inches of sand on top of that. Hoping that it will discourage them from digging and they will hopefully use the designed tunnel system. If not I also thought about cutting places in the egg crate and putting tupperware container for them to di down into, making sure they couldnt escape under the egg crate?
Ideas? Any photos of your setup?