Recurring Stagnation Of Water

[Clownfish_Boy]

has any animals been lost in this condition/ or do they hover at the top during the morning hours before lights on

Yes, when it happened before (some time last month) I lost a clownfish and an angel. I attributed it to either poor oxygenation or Hydrogen Sulfide

Can we see a video of the surface movement? When you say hydrogen sulfide, do you mean it just stinks, or it stinks exactly like rotten eggs? Hydrogen sulfide has a very specific smell, not just a general stink stank stunk kind of smell.

Like rotten eggs. The HOB Marineland filter agitates the surface.

 

[Rubberfrog]

Yes, when it happened before (some time last month) I lost a clownfish and an angel. I attributed it to either poor oxygenation or Hydrogen Sulfide

Like rotten eggs. The HOB Marineland filter agitates the surface.

Look for an area that can be hosting anaerobic bacteria (an exceptionally low flow area). In the meantime, leave a couple of eggs in a box under your bed. See if they can match the smell after a few weeks.

 

[Clownfish_Boy]

I was thinking the most likely cause would be a dead spot; but just don't see how I could have one with those two big powerheads and the Marineland 400 filter.