Well, just to come at this from a different angle IMO the down tube is too large - I'm pretty sure i saw somewhere it is 2.5"
I have a chart somewhere (can't place it at the moment) but the rule of thumb from my diving days is that the *smallest* visible bubbles rise at about 1 foot per second. So if you look at a flow vs velocity chart on 2.5" pipe you need about 18gpm for a 1FPS velocity (round it out to 20gpm for a nice number) Thats 20gpm just to keep the *smallest* bubbles static in a column of water. So realistically you're going to need 30-40gpm to blow everything down and through - which you're not going to get through a 3/4" injector..... So you'd need multiple injectors..... And bigger piping...
I think I saw the skimmer was 5' tall. So if you add that head to the head loss across the injectors and friction loss, I'd guess you're around 15' of head (or probably more). That's a pretty skookum (read inefficient & noisy or expensive) pump.
2" @ 1FPS = 12gpm
1.5" @ 1FPS = 8gpm
Anyways, just my thoughts to chew on for what they are worth.
Jim
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