Isn't it interesting that we believe that MTS, which is based on dirt or unknown composition, is "good", but, old substrate, which has miscellaneous organic stuff mixed in, leaving a substrate of unknown composition, is "bad". Personally, I think if I can stir up the substrate a bit and it stinks bad, it is "bad", but if it doesn't stink, it is "good".
hahaha! The debates and concerns will never end on this one.
I have a single tank that has the same substrate (without deep cleaning) >30yrs wet. I've added finer grain material in a layer to increase the depth a couple times once I added plants to my tanking but the original river gravel still sits on top of decades old and unused UG filter plates.
I use it as a breeding tank.