I would not do it if you plan on adding fish that didn't come from the same water supply... there can be a ton of creepy crawlies in that water. Not just parasites, but bacteria too. One little bird dropping disolved in that stream and your fish could end up toast. Be very careful. Its probaly 50/50 that you'll be fine, but you can get play sand or pool filter sand for $3.00 for a 50lb bag at home depot. I wouldn't risk the potential for contamination with unknown biological hazards.