So I killed some fish via O2 depletion.....

[joelespinoza]

I feel fairly bad about this, but it was still a somewhat enlighting experience, so I thought I would share it.

I have a 10 gallon... I will call it a quarantine tank (very loosely). Its sitting on my kitchen counter, it has a bunch of macro algae and 4 pinfish from the gulf of mexico along with a couple grass shrimp some mexican turbo snails and some hermit crabs. It has a small ~70-80 gallon powerhead and an aquaclear 70 (400 GPH). Assuming the aquaclear flow is reduced some lets call the whole turnover 400 GPH, thats 40x tank size. I have had this setup going several weeks now.

All the algae has been growing, the algae on the glass and the macro algae, I have been feeding fairly heavily (couple pinches / day) and I am sure the nitrates are going up a bit, but I wasnt worried about it too much. This morning I came out right after the lights came on and 2 of the fish were dead, ther other 2 were laying on macro algae gasping for breath, the snails and hermits seemed unaffected, they were climbing on the glass and doing their thing like normal, did not seem stressed in the least. There was no power flash or flicker last night, flow was going on all night same as normal.

So from this accidental experiment a couple observations:

Smallest and largest fish died from lack of O2, medium 2 survived. (smallest one looked like it had been dead a while, larger more recently)

Inverts dont seem to need as high an O2 concentration as verts (fish)

Macro algae can consume a LOT of oxygen when the lights go out.

It made me wonder about people with macro algae filled refugiums, and how much they deplete the O2 when the lights are off. I know there was a discussion here about algae screens dying when the power goes off, and poisoning the tank when power comes back on, but that effect would take a relatively long time, perhaps days. However, noticing this effect, even with TONS of flow in a 10 gallon, I wonder how much dies off in a macro algae filled refugium even during a relatively brief period of power outage, and how much it ends up polluting the display tank when the power comes back on. It made me glad that my algae isnt in the tank when my power goes off.

[Ace25]

I am confused. How are you coming to this conclusion? Did you take ORP readings to determing the oxygen levels? Plants breath in CO2, the exhale O2, so the more plants you have the more oxygen will be in the tank. To top it off, you have a HOB filter that is creating surface movement which will also speed up gas exchange. I have a feeling there is some other cause other than O2 related that caused the fish death (like possible an ammonia spike from the first fish, possible the largest one, dying). It is only when algae dies where you can get low O2 levels, this is because of all the dead matter in the tank and the anerobic bacteria in the tank going into overdrive to get rid of it, which will lower O2 levels.

[kerry]

What did your nutrient and supplement levels read? You should have plenty of surface agitation to oxygenate the water so I would not think the algae sucked it all up. Without getting all the test levels of the tank measured and recorded its hard to know for sure.

[Ace25]

Here is a quote I found to explain night time scenerios.

Like most plants, many algae produce oxygen during the daylight as a by-product of photosynthesis. At night these algae consume oxygen, but usually much less than was produced during the daylight. Many common situations, however, can reduce the amount of oxygen a bloom produces without reducing its nighttime oxygen demand. Extremely calm or cloudy days may reduce photosynthesis and oxygen production. This type of oxygen depletion may kill fish directly or weaken their immune systems through prolonged stress.

[kerry]

I am confused. How are you coming to this conclusion? Did you take ORP readings to determing the oxygen levels? Plants breath in CO2, the exhale O2, so the more plants you have the more oxygen will be in the tank. To top it off, you have a HOB filter that is creating surface movement which will also speed up gas exchange. I have a feeling there is some other cause other than O2 related that caused the fish death (like possible an ammonia spike from the first fish, possible the largest one, dying). It is only when algae dies where you can get low O2 levels, this is because of all the dead matter in the tank and the anerobic bacteria in the tank going into overdrive to get rid of it, which will lower O2 levels.
Algae will consume O2 in the dark but I doubt enough to over come the exchange . Guess I was writing the same time you were Ace25, then the phone rang. Pretty much the same post.

[kerry]

There we go again!! LOL. We have to stop meeting like this???

[Ace25]

LOL, yes, we seem to be on the same page with this. ;)

[kerry]

Unless he had a green water explosion in the night I dont think it was 02 related.

[SantaMonica]

Nighttime respiration is only a few percent of daytime photosynthesis.

[joelespinoza]

A couple weeks ago (when the macro and fish were MUCH smaller) I did not have the Aquaclear in there. I just had the small powerhead and a airtone driven sponge filter. Every morning the fish were having more and more trouble breathing, but after having the lights on a couple hours they were fine. I tried leaving the lights on overnight and the next morning they were fine, no gasping at all.

Ever since I added the aquaclear I have not been paying that much attention, thinking the surface agitation should be plenty, but the fish have grown a LOT (easily 4x the size) and the macroalgae pretty much takes up the whole tank now (or did this morning before I took 90% of it out).

I will check the nitrates when I get home, they could easily be contributing to the problem, however the hermits, snails and shrimp looked fine this morning when 2 of the fish were dead and the other 2 looked on deaths door. An hour later the nearly dead fish were fine, eating and chasing each other all over the tank. From what I understand if nitrogenous waste product were killing fish the inverts would have long since been dead. It also does not make any sense that nitrates would be killing the fish ONLY when the lights are off. If anything the reverse should be true, the tank warms up during the day by a few degree from the lights, which means the oxygen saturation level of the water should be higher at night and lower during the day.

There is algae on the glass, but the water is perfectly clear (much clearer than my main tank) the amount of chaeto and calurpa I threw out this morning was probably 1.5x the size of a bastketball. I will measure the nitrates when I get home, but honestly with the symptoms I am experiencing I think the main culprit is aerobic repiration on the part of the algae at night.

[Ace25]

I will check the nitrates when I get home, they could easily be contributing to the problem, however the hermits and snails looked fine this morning when 2 of the fish were dead and the other 2 looked on deaths door. An hour later the nearly dead fish were fine, eating and chasing each other all over the tank. From what I understand if nitrogenous waste product were killing fish the inverts would have long since been dead. It also does not make any sense that nitrates would be killing the fish ONLY when the lights are off.

Huh? Did someone mention Nitrates in this thread and I missed it? Without having any idea on what your O2 levels actually are you are just taking wild guesses and the cause. In regards to what I mentioned, ammonia, it is true that inverts are usually the first to go when ammonia spikes happen, but who knows the whole story. It could be "large fish 1 died, after 3 hours released a lot of ammonia, fish 2 was already very sick but visually looked ok, gets hit with ammonia, and croaks as well", but that is just another wild guess like saying the O2 levels are depleated. High ammonia will also make fish "gasp for air".

[kerry]

It is kind of strange, check/test all the levels, they should point to something.

[kerry]

Huh? Did someone mention Nitrates in this thread and I missed it?
I mentioned to check nutrients and supplement levels.

[joelespinoza]

High ammonia will also make fish "gasp for air".

High nitrogenous waste products cause a lack of O2 binding to hemoglobin, which results in a similar response in aquatic animals to very low O2 conditions. However I have never heard any reasons why ammonia, nitrite or nitrate toxicity would be VERY suddenly and visibly be relieved by the presence of light (symptoms obviously relieved within 10-15 minutes and totally gone within 1.5 hours).

I also highly doubt that ammonia or nitrite is an issue here, this tank has a layer of aged live sand and some live rock pieces, and has been setup well over a month. The sponges I am using for filtration also came directly out of my 55 gallon.

I will check the Ammonia, Nitrite and Nitrate when I get home, and would gladly check the O2 levels, but A: I dont have an O2 meter, and B: I am betting after removing 90+% of the macro algae the problem will be gone. I could be wrong, but the light factor just seems too overwhelmling to me.

[joelespinoza]

Nothing, No ammonia, no nitrite, no nitrate, PH is 8.3-8.4

[kerry]

where is Alk, Ca, and PO4.?

[joelespinoza]

where is Alk, Ca, and PO4.?

You really think Alk, Ca, or PO4 being slightly off would cause the death of fish when the lights are out?

I can test the Alk and Ca if you really think it would make a difference, but I dont have a PO4 test kit. The couple times I needed one I borrowed it from a friend.

[Garf]

You really think Alk, Ca, or PO4 being slightly off would cause the death of fish when the lights are out?

I can test the Alk and Ca if you really think it would make a difference, but I dont have a PO4 test kit. The couple times I needed one I borrowed it from a friend.

Not a chance! No way! If that were the case we would all be in the crap.

[kerry]

You really think Alk, Ca, or PO4 being slightly off would cause the death of fish when the lights are out?

I can test the Alk and Ca if you really think it would make a difference, but I dont have a PO4 test kit. The couple times I needed one I borrowed it from a friend.
No, not saying that. Just trying to get a better picture of your tank.

[joelespinoza]

Ever since removing the algae I dont have any issues anymore. There is no more gasping in the mornings or even if I leave the light off all day.