I bought two Hydor Koralia magnum 8's. One is on the back left side about 8" up and I have it pointed up. The other one is on the right side in about the same position...it points to the front of the tank and up as far it'll go. It provides dead enough areas for the fish to rest, but also stirs enough water that poo doesn't last very long.
My tank is 450g, 8' L x 3' W x30" high. My biggest concern was they wouldn't stick to the 1.25" thick acrylic, but the magnets have suction on them, so no worries.