I think you're going to need something like
this, which is what I have connected to my tank. Then I just have some hose connected to the barb, and that connects to my aeration wand.
I have one similar to that, also green, but it adjusts from
1/32 l/m to
8 l/m. I've only been using between 1/32 l/m (yeast starters) for 4-8 minutes and 1/8 l/m (5.5 gallon batches in a 6.5 gallon brew bucket) also for 4-8 minutes.
That's what I would look for.
1/2 l/m is way too fast, causing large bubbles on the surface, which is O2 that didn't get dissolved, and is lost to the air. A mere rippling, breaking the surface, is plenty fast already.
Now... 20 cu.ft. of O2 is a large volume (28.3 liters per cu.ft.), so even wasting 3/4 of it may not be a big issue. I reckon at my frugal usage rate, that 40 cu.ft tank will last around a lifetime of oxygenating wort.
