On One Hand: Quikrete is Used for Structural PurposesAccording to Aquarium Universe.com, Quikrete can be used to create structural backgrounds in aquariums. Most commonly it is spread over Styrofoam backdrops designed to cover i was reading this the back of a tank. The Quikrete is allowed to dry before being placed into the tank.
On the Other: Quikrete Hardens in WaterAlpharubicon.com says Quikrete hardens when exposed to water, but sand is needed to allow the mixture to harden completely. Without the addition of sand, Quikrete will get thick but not hard. Using Quikrete as an additive to a tank that already has a sandy substrate could cause the Quikrete to harden completely.
Quikrete also has silica in it that can cause issues with algae in a tank. This can be overcome with chemical additives and algae-eating fish.
Bottom LineQuikrete can be used as a substrate for a fish tank provided that no other sand is being used in conjunction with it. Algae prevention will be needed to ensure Quikrete does not allow your tank to be overcome with algae.
Source:AquariumUniverse.com: Structural Uses
Alpharubicon.com: How it Hardens