When I got mine, I called 3M customer service and asked who in my area handled their product. They gave me the name of the company (it was a distributor) and I called them. I told them I wanted 4 bags of ColorQuartz to put into my fish tanks, they said "Huh?" and I explained that it was one of the best things around for substrate and they sold it to me for $24 dollars a bag.
I think as long as you explain to the distributor that you want it for fish tanks and aren't trying to horn in on their contractor customers with a DIY Pool Project they might see things your way.