...If I understand everything, and I'm reading the below chart correctly, the 80 should push ~2CFM at 4'.
If my diffusers are as efficient as the XL1 (they wont be) then I gather they'll push 1400GPM total, since the 2CFM is divided between the two @ 700 each.
I'll estimate that I get maybe half that from my homemade diffusers. That should turnover my 90,000 gallons in just a little over two hours.
That sound right?...
You have read the Hp-80 curve correctly...2 CFM at about 2.5 psi back pressure. That's 2 psi for your diffuser depth and 0.5 psi (conservative estimate assuming appropriate sized hose (3/8" or 1/2") for line and plumbing affects. The shorter your air lines the better with respect to reducing back pressure.
I doubt that any homemade diffuser that I could build with a drill and PVC pipe would come close to the Vertex specs, but that won't hinder your setup with respect to pond turnover because your pond is so small and your 80 pump will be over sized. I would say that your estimate of half of the Vertex efficiency is a good guess.
IF you were to achieve 1400 GPM from your total set up...you would divide 90,000 gallons of pond water by 1400 GPM which yields 64 minutes. That's just over an hour. That seems like a very fast turn over and I would be concerned with muddying up the pond which, in your older pond, means potentially stirring up the toxic muck. This is just a concern as I have little experience with aeration startup on small older ponds. My pond is 500,000 gallons and was completely renovated before aeration went in. You may find that the 80 is too much pump for your liking and, in that case, you would have to put a valve in to dump some of the excess air out to the atmosphere or get a smaller pump. To be honest, your fountain may be doing a pretty goo job already...if it has a deeper intake. Do you know the GPM for the fountain pump?
If I were you, I would take a very conservative approach to starting up your system. Put the diffusers in shallow and away from the bank, maybe 18" below the surface in at least 4' of water (5, 6, 7' would be better). This will reduce the water movement (GPM) which, in turn, keeps the water velocity down which, in turn, reduces the lifting of silt off the bottom. As you can see from the Vertex chart...the water movement reduces at a greater rate the closer the diffuser gets to the surface. So, we don't know what 18" depths will do for you, but better safe than sorry. You can always move them deeper once you understand the effects.
To understand the effects...Watch during start-up for the water getting muddy and smell for sewer stink. After a significant run time has been achieved, you can measure the water temps from the deepest part of the pond and one foot increments and understand if the water is getting fully turned over. The water temps will all be very close to each other (probably within a degree or two) if full turnovers are taking place. Other parameters will also be consistent, like PH, DO, TDS, etc. Make sure you take these temps prior to the system going on line so that you know where it all began.
Start developing your start up plan and share with the forums. Those with similar conditions to yours will be able to critique it better than I.
