These are one the best air stones out there that money can buy. They are weighted to sink naturally and can be taken apart if they ever were to clog - so they can be cleaned or the disks replaced.
They also work great inside Sponge Filters. Instead of replacing a ceramic air stone every every few months as they get worse and worse for bubbles, install one of these once and nearly never worry about it again. Tighten or loosen the disks to change how big the bubbles are.