You can use sand in your tank, but some kinds (aragonite/calcium sand) will raise pH which you may not want with freshwater fish (I'm assuming you are going to be keeping angels in it based upon your username? ) Pretty sure that silica sand is one 'white' kind that won't have an effect on pH, so could be used in a freshwater set up. You can normally find it in places like Placemakers, Mitre 10 etc, and it's relatively cheap too which is a bonus. However a word of warning, you may find it hard to keep clean, poos sit on the top of it and it can get dirty very quickly! The fish colours may also look a little washed out against it as it is quite pale, but it's totally up to you to decide what 'colour scheme' you want for your tank . Hope this helps!