To be completely honest I dont know the exact effects, but common sense tells me that the fish will die from what can be a number of things meant for salt water tanks in the marine sand. Why take the chance? There are many freshwater sand ideas that are completely safe such as super naturals aquarium sand which is completely white and I provided a link to in the sources area. You could also use pool filter sand which is a very cheap alternative. I believe you can get around 50lbs for $5.
