Pool filter sand is a heavy, larger grain sand that will not do this and is very inexpensive. It just needs to be well rinsed before using. If you still plan on 'mixing' sand with gravel, just keep in mind they will not 'mix'. In no time, you will wind up with all the gravel migrating to the surface and the sand beneath. It makes cleaning quite difficult, too. Plants do perfectly fine in just pfs as long as you use root tabs for heavy root feeders.