See the question 'None of my Triops hatched out' You don't need pure water but you will get more Triops hatching if you do. Triops in the wild come from temporary pools of rain water, so they have developed a special survival technique. A pool of rain water may dry out at any time, so to ensure that Triops hatch out in optimal conditions, the egg senses the amount of minerals dissolved in the water. The less minerals the fresher the water must be. A large pool of water will also stay fresher for longer than a small puddle for example. See the Triops egg page for more detail.