The block from the pet store is great to use, I just didn't know thats what you meant when you said potting soil. I got a brick once but never used it because expanding it was too much work for me! But if you do ever have problems with humidity then vermiculite mixed in works great, it absorbs tons of water.
I had about 50 different tarantulas (around 30 different species) and 20 or so scorpions (4 different species) and a handful of other arachnids/insects. It just became too much work for me to care for them and the snakes, plus I traded a bunch of my tarantulas for snakes.
I still have one tarantula, he matured out over 2 years ago and is still hanging in there! Hes a Grammostola rosea - rose hair tarantula