Don't use it. Most of it is power fine and will be a huge mess when moving things around or cleaning. Use pool filter sand. Your limitation is just going to be color with it. Or use the diamond blasting sand mentioned for a dark color.
Pool filter sand will usually come in white or a light tan. You hardly have to rinse it before putting it in the tank because it's already clean for its intended purpose in the pool. You'll have a diatom breakout a few weeks in without a doubt as it (and play sand) will have more silica in it. It will be ugly but just wait it out. I used it for my FW
tanks. At $7-$9 a bag it's worth the money and a huge savings.