If you are getting an air pocket accumulating in the upper curve of the U tube - then you don't have enough flowrate.
But at the same time - you may not be getting enough flow because your drain line is sumberged too far down underwater in your refugium. This "may" be causing some backpressure ( for lack of a better word ) and not allowing the drain to vent and empty fast enough.
The overflow works on a principle of two different water levels.
The inner box ( tank side ) must be slighting higher than the water level in the outer box. If this condition exists - then the water will be pushed up and through the U tube by atmospheric pressure. Once the water's in the outer box - it will natually "fall" down through the drainline by gravity.
If the drainline is submerged too far underwater - this can create a situation where
1) The drain can not vent to top.
2) The water can "back-up" in the drain, again causing venting problems and making it harder for the water and air to discharge.
Try raising the end of the flex drainline up so the water discharges just at, or just barely under the water suface in your refugium.
Your 29 gallon tank measures 30x12x18.
I would guess that the Amiracle has a standard 1" diameter U tube.
Ideally - this would allow around 600-700 GPH flow.
But ..... at that rate you'd be turning over the water around 20 to 24 times an hour. This is excessive in my opinion, especially for a 29 gallon tank. It's just too much to be running to a fuge.
Ditch the idea of running the Magdrive 9.5
That pump is way too much for your set up.
Ideally ... if you're interested, I would suggest you take a different approach underneath the tank and move towards a Magdrive 7 and do some plumbing modifications.
If you placed the Mag7 in your sump, and tee'd off the return line with a tee and smaller hose - you could borrow some of the flow to feed your fuge. Allow the fuge to overflow back to the sump through the bulkhead as you are now.
Run your drainline directly to your sump.
Obviously run the main hose from this new tee on up to your tank spraybar/fitting.
Keep the ball valve on the pump, and also install a small ball valve on the new hose leading to the fuge.
Run your drainline flex in a way that allows it to vent .... meaning the open end in the sump is just at the water level in the sump ( or just slightly underwater ).
This should improve things a lot.
You can keep your Rio2500 if you want to .. although I don't care for them too much, and there have been many problems reported by those that use them.
Short term - fiddle around with repositioning the drainline you have now.
Long term - consider re-doing the plumbing underneath.
You want sufficient flow through the U tube to forced out entrapped air bubbles - or they will form the air pocket you're now seeing. This MUST be corrected or your U tube / Siphon will fail.
but it is a hangon... 6 1/2" long L to R.. 4" wide and 10 inches tall
