They say this so that you'll have more success in hatching out the Triops. Triops like water with low mineral content in order to trigger them to hatch out. By adding things to the water like sand, plants, ornaments then you could possibly reintroduce minerals or chemicals to the water which will cause the Triops eggs not to hatch. You'll find using a substrate like coral sand that has been very well rinsed in a bucket is fine. Just ensure you never use soap or any detergent in any of the tank items. You'll kill the Triops.