Here we go again: MIs, Take III

[George]

I think (angel) fish dead overnight has some reasons other than oxygen. For a couple year, I ran a tank that has no skimmer. The tank had some very sensitive angelfish like golden and regal among other angelfish. All fish looked healthy and happy. As a matter of fact, one night I forgot to turn on my power heads after a maintenance. All fish were ok the next day morning.
I since upgraded that tank to a bigger one with a skimmer. But I am not hesitate to run a angelfish tank without a skimmer again. The only reason I wanted a skimmer in that tank was because I wanted to keep the water clean in order to keep some corals.

[Aquattro]

Honestly, even with O2 as a strong suspect, unless we monitor full time and chart values and find a sudden dip coinciding with a fish death, it's still mysterious dying fish syndrome.
The 2 in town that died. Tank is stable, nothing of any significance changes day to day, no power outages, etc. Fish survive fine for 6 nights, both die on 7th. What happened that could drop O2 to lethal levels on that night and not the others? Maybe something. Maybe not.

I did lose a MI and 2 large angels in a confirmed low flow setup overnight. I now have 2 MI doing well in a tank with surf conditions. But unless I spend a grand on an Apex DO meter and log O2 variances, we can't be sure what it or isn't happening.

[Samw]

In my case, it was reproducible. Flame angel a couple of times. Lemonpeel angel one time. Cherub another time. Always established for many months or years. Always after skimmer off overnight. Never a problem otherwise. If I had to guess, my DO level would have been around 20% (maybe much less. Eric's clownfish tank experiment had DO levels of 15%) without a skimmer running at night with a tank with algae.

Not saying this can be the only reason for mysterious death. But it is overlooked.

By the way, what was the reason for H2O2 dosing? To deal with an algae problem?

[Aquattro]

Originally Posted by Samw

By the way, what was the reason for H2O2 dosing? To deal with an algae problem?

Not sure exactly. General cleaning, I think. I confirmed with the tank caretaker that it's dosed a couple hours after lights come on, so no evening impact.

[Samw]

Ok, so what I was trying to get at is if the dosing was due to an algae outbreak and whether the dosing had just started and algae started to die. Lots of algae, low ph, higher temp (my apt reached 30C last week), decaying matter all consume oxygen at night and was looking to see if the timing lined up with the deaths on the 7th day.

To be clear, I'm not saying people can't have skimmerless successful tanks with angels. Tanks can be aerated by means other than skimmers. That just means those skimmerless tanks didn't have hypoxic conditions. It doesn't mean other people's tanks don't have hypoxic conditions for some demanding fish on a bad day.

Anyhow, I know I'm beating a dead horse and have not been convincing enough. Maybe I'll have more to add when I get a chance to do some experiments on my own tank and intentionally lower my DO in a controlled way to see how my fish will react. Anyone know any biology students at UBC with access to a respirometer that want to do some research on reef fish hypoxia?

[Aquattro]

Originally Posted by Samw

Ok, so what I was trying to get at is if the dosing was due to an algae outbreak and whether the dosing had just started and algae started to die.

Ah. No, I'm told that there is about as little algae as you can get in that tank. The owner really doesn't like algae I think he has one little strand of bryopsis.

[Aquattro]

So back to the saga that is MIs. Today they started a bit of swatting at each other, so I may have to break the set soon. Not sure which one to keep, tough call. Big one is fatter, eats mysis better. Small one is still a bit skinny, but loves pellets and fits tank better. Also not keen to rehome one to hear it died a week later. In hindsight, ordering a spare might not have been #1 smart idea.