Mastercraft All Purpose Sand can be used for fish tanks, but it is not ideal and requires intensive preparation. Because it is a construction-grade sand, it contains high amounts of fine dust, can alter water hardness/pH, and may contain sharp jagged grains.
While Mastercraft is not inherently toxic, you must manage several risks to ensure your fish remain safe:
Toxicity Check: Always double-check the bag to ensure there are no added chemicals, such as fungicides or mold inhibitors, which can be lethal to fish.
Thorough Washing: You must wash the sand repeatedly in a bucket until the water runs clear. Otherwise, the fine dust will cloud your tank for weeks and could get stuck in your fish's gills.
pH and Hardness Changes: This sand is typically silica, which is usually inert. However, construction sand can sometimes contain crushed minerals that raise your water's pH and hardness, making it unsuitable for soft-water fish.
Compaction Risks: Because it is very fine, it can pack down tightly. You will need to stir it occasionally or keep bottom-dwelling fish (like Corydoras) to prevent toxic anaerobic gas pockets from forming.
Better, Safer Alternatives:
For a smoother, safer aquarium experience, it is highly recommended to use either Pool Filter Sand or CaribSea Aquarium Sand. These options are pre-washed, perfectly sized for optimal water circulation, and have smooth grains that won't damage the slime coat of bottom-dwelling fish.
Well - one thing that i wonder about is they claim it is safe for sand boxes; historically sand that is safe for little kids is safe for fishes though sometime these claims are made arbitrarily. After all how many 3 or 5 year olds have gotten sand in their mouth.
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When i was a kid we didn't buy play sand - we tore out a bomb shelter our home had and used the sand for the sandbox