Ok. The oil in the mysis supercharges the sump return pump, it runs faster, it puts more water the main tank, and the sump lowers accordingly.

Ok I'm just being silly. But maybe there's something to this. I.e., the water properties have changed "just enough" maybe there's less friction and thus the overall flow through that pump is just slightly higher?

Ok NOW I'm out of ideas. :lol:

:crazyeye:

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I'm starting to get it.
The sump is baffled so that the level on the skimmer side, also the drian side, doesn't change. The overflow is a 1.5 inch 90 degree elbow with a DYI strainer that determines the water level. When the mysis is added and the water level rises the strainer is completey submerged. Sump level on the other side of the baffles lowers because a) the skimmer has less air and is more full of water. b) tank level is higher.

I think we're on to something with the fatty composition of the food. Could it be creating a back pressure in the drain like you said above? ie. An oily drag on the water going down the drain, thus decreasing drain flow?

What a wierd phenomenon.

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That sounds more plausable Tony, fatty acids coating a drian pipe wouldn't slow down the drain, they would speed up the return pump. I could run an experinement of the flow rate before and after adding mysis to test this theory. Good Job! :biggrin:

I think that the change is happening in the skimmer. Any small change in the water properties changes the volume of water currently in the skimmer. You are basically removing/adding water from the sump/tank by changing the volume of water in the skimmer.

actually, the water level in the skimmer shouldn't really change but the bubbles will stop. how much mysis are you feeding? there should be no real change in any level but that is wierd. any amount of mysis getting caught against your ''strainer/overflow'' will change the level. actually, that strainer would worry me a bit. any dead little fish or algea or anything getting on that strainer could cause flood issues no? I had this setup on my 90g [1 strainer] and after a small flood I drilled 2 outlets with boxes.

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You're right, the skimmer level doesn't change much, just the bubbles. Every time I feed I check the overflow and its clear, which is why I'm still leaning toward Tony's theory about the pump.