How will you know what to test for though?
Regular aquarium test kits will only tell you if its effected the PH and stuff - it may well leak something toxic to fish that the tests are not designed to pick up
The other thing is magic sand when wet clumps and sticks together so if your fish got a mouthfull of it its like a lump or clot of sand not just grains that s/he can spit out - I just cant see it being a good thing.