I'm not sure about the cheapest, but I have for the last two years sold Vertex Water Features diffusers. Check it out at on their website with the same name. It is under warranty for 5 years. It is easy to sink and comes with 2-9" disc. The upright base keeps it off the bottom and it has very small bubbles. I have had no trouble with them at all to this point. Suggested Retail is $115.
