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

Sump Design

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.

Sumps attached and with water flowing

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.

  • $\begingroup$ I've read your question three times and still don't understand the problem. "I have a pump ... and the return ... both flowing at an equal rate." What goes into a pump must come out. What exactly are you trying to say? It sounds to me that there isn't enough water in the system and that the pump is draining the right sump. Maybe draw in all the connections and water levels to clarify. "Two 32mm flexi-pipes connect the two containers." How does this compare with the pump hoses? What is the pump rate in L/s? $\endgroup$
    – Transistor
    Jul 25, 2020 at 12:21
  • $\begingroup$ Hi Transistor, this is an aquarium sump. Hence a pump sits on the far right and goes into my aquariums (which sit above the sumps on a cabinet) and then an overflow drain for those aquariums comes back into left hand sump. For simplicity imagine my pump just goes straight from far right into far left chamber. Regarding flow rate - I have a variable pump which can run up to 7200lp/h - but i'm running it now at somewhere nearer to 1000lp/h. $\endgroup$ Jul 25, 2020 at 12:54
  • $\begingroup$ At what height are the two connecting 32 mm diameter flexi-pipes? Are they horizontal, or are they sloping - the exit from tank 1 is higher than the entry at tank 2. If so, is the entry height at tank 2 the same as the height of the first baffle in tank 1? $\endgroup$
    – Fred
    Jul 25, 2020 at 13:37
  • $\begingroup$ Hi Fred - connector flexi-pipes are both horizontal, and positioned 100mm up tank - entering and exiting at same height - baffles immediately to left/right of connectors are at 300mm. I'll add a photo of the setup for real to clarifying anything else that may be confusing. $\endgroup$ Jul 25, 2020 at 13:42
  • $\begingroup$ Tip: use @username to send a message to the user's inbox. $\endgroup$
    – Transistor
    Jul 25, 2020 at 14:47

2 Answers 2


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.

enter image description here

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.

  • $\begingroup$ Please see 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. $\endgroup$ Jul 25, 2020 at 15:11
  • $\begingroup$ OK. I think your question is, "Is a 50 mm height difference a reasonable head for two 32 mm diameter hoses at 1000 L/h?". See the update. $\endgroup$
    – Transistor
    Jul 25, 2020 at 15:49
  • $\begingroup$ Thank you @Transistor for helping me understand the relationship between bore size and flow - since the bore size of the bulkhead cannot be changed (without me expanding the hole in the glass) would it improve the situation to massively increase the flexi-pipe size e.g. 50mm internal diameter pipe? Or would the bore size of the bulkhead continue to restrict? $\endgroup$ Jul 25, 2020 at 15:56
  • $\begingroup$ Try using the calculator for 25 mm fitting (say 0.040 m long) and various sizes of pipe of fixed length. (Don't forget to add the fitting pressure drop twice for each hose.) I think you'll find that the fitting is going to be the problem. You could raise the first tank 25 mm to improve the head but you'd need to make sure that the lower tank doesn't overflow when the pump is stopped. $\endgroup$
    – Transistor
    Jul 25, 2020 at 16:10

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 ?

  • $\begingroup$ Hi blacksmith - I do appreciate a need for head pressure in a more general 'pushing the water through' situation, but my understanding of the two connected sections (where the baffles are at exact same heights) is that they should - assuming the pipes can handle the flow - be acting as if they are one uniform body. Given the pipes clearly can handle the flow (the system is running in a stable manner) this is where i'm a bit dumbfounded. I feel like your answer is potentially correct and 50% of the way there but I would like to understand the various forces in action a bit better. $\endgroup$ Jul 25, 2020 at 15:18
  • $\begingroup$ For example in this post: physics.stackexchange.com/questions/491559/… they indicate: "If the water flow through Pipe 1 is NOT more than 2x greater, than the flow through Pipe 2, then the tanks will always have the same water level" - it's not entirely identical but i'm not seeing how it is dissimilar in a manner which relates to this. $\endgroup$ Jul 25, 2020 at 15:21
  • $\begingroup$ I forget if it is thermodynamics or Newtons laws ; Basically "nothing moves unless it is pushed". $\endgroup$ Jul 25, 2020 at 21:16

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