If you're buying sand for reptiles be sure theres nothing added,Ikeep reptiles too and alot of sand that is for reptiles is called calci sand,as there is alot of calcium added to it incase of ingestion which happens alot with reptiles.The sand is normally coloured though,And the die does run in water ive had water bowls in all colours when a snake is shedding as its in and out.You can get kilm dried sand i don't think anything is added to this but check and it won't be died.That's my experience with reptile sand,but if you do by reptile sand just check nothing is added that may harm the fishes.Much cheaper to do what vbaby and penn said,have fun.