Unequal water level in connected containers - Aquarium Sump

Question

I have recently designed and had fabricated the following two sumps for my Aquarium (See image).

I have a pump in the right hand container (section 4), and the return is coming back into the left hand container (section 1) - both flowing at equal rate (~1000lp/h).
Two 32mm flexi-pipes connect the two containers.
I was expecting the water level in both containers to equalise, but the left container is consistently running near to maximum whilst the right container water level is - as I would expect it to be - at the baffle height for 'section 1'.
Why am I not seeing equal water heights in both?
See below a picture of the situation occurring with RED lines indicating water height, BLUE lines indicating baffle heights, GREEN arrows indicating water flow direction. Please ignore the fact that I currently have one of my bypass valves turned on - it's all I can do to drop the water height in the left container a few centimetres.

Note - Reducing flow rate, and removing water from the system does not seem to have any positive effect on the left hand container. Filter sponges/socks etc were added after this effect was happening. The system has been running stable for 24 hours with this effect visible.

Transistor · Accepted Answer

The pressure drop between the left and right sumps can be calculated by measurement. For fresh water 10 m = 1 bar (approx.) so 10 mm = 1 mbar (or just work in mm).
There are a several useful calculators online.

Figure 1. Resuts from Copely's flow rate calculator. Click for 100% magnification.)
I've guessed that the pipe length is about 300 mm and that while the hoses are 32 mm the fittings through-hole is only 25 mm. I've estimated the height difference between your two tanks at 50 mm.
According to the calculator one hose should be passing 9 L/min. You have two so that's 18 L/min at 50 mm head.
18 L/min = 1080 L/h.
I think everything is as expected.

blacksmith37 · Answer

There must be pressure ( water level) difference to provide force to push water through the system. If you stop input and output for a few minutes the levels will even out , except where a baffle may prevent it .With no flow ,no pressure differential is need to move water . It is not much pressure , height of 4 " equals about 0.17 psi. Unrelated , have you considered a light to grow algae or plants like a salt water refugium ?

Answered by blacksmith37 on June 4, 2021

Unequal water level in connected containers - Aquarium Sump

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