Dick,
For your application the Gast pump should work. Even at 12' of depth the compressor will have to generate 5.3 Psi to get to the depth. I would assume you would only require 100' of tubing to feed your diffuser. So your total psi required should be 5.49 psi.
Next I would check the pump curve to ensure you have enough cfm of air to feed the diffuser you will be using at 5.49 psi.
To compare the Brookwood, it will provide 1.5 cfm at the 5.49 psi. The CoActive AirStation will require only 1 cfm so this is a perfect match.
I was surprised too to hear a price of $40 for a compressor. The Brookwood cost $325. I have put together a turnover calculation sheet that I will email you for your review.
