I've just changed to sand, after years of gravel substrate. I poured the full 15 kg into a container, then filled it up with tap water, i the agitated the sand. i repeated the process several times. then i scooped up the sand in a small container. this i put, slowly in the tank. i then poured the sand, from the container (keeping it no more than an inch or so off the tank bottom) where i wanted it. i did this, until all the sand was used. the water did not get too cloudy. also any (floating) debris was scooped out with the same container. the whole thing took 1 hour, from start to clear water.