Substrate?

[PitOnTheProwl]

Not much at all, just enough to make it slightly crumbly. I would say about a 1/4 to < 1/2 cup of water, to about an inch deep layer in an exoterra 20 gal long tank.

Not adding enough. I buy the bricks and it takes a gallon to a gallon and a half to blow it up.
Even if you are buying the bag stuff it will hold a lot of water. Try adding a full cup or two and make sure you mix it very well.