The strangest thing

Wilhelm

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I have a 120 gallon reef tank. The back wall, over the last two years, has become coated with a thick mat of algae. For no given reason, I suppose except finally decided it had gotten unsightly, I decided to brush it off. I was surprised to see a lot of residue come off the back wall along with the algae. My fish, all of a sudden, huddled all together in one corner of the tank. I thought they were panicking and stopped cleaning after doing about 25% of the back wall. Whithin a couple hours a recently purchased Bird Wrasse and a two year old Hippo Blue Tang were dead and my Melanurus Wrasse, Clarki Clown and a Blue Fin Damsel were very lethargic. My pajama cardinals and my Yellow Tang were hiding.

No further fatalities the day after the incident. What happened?

Water is tested weekly and I change 30 gallons every two weeks.
 

Wilhelm

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I am not sure. This was a back wall not the sand bed. Two days after the incident the survivors seem well but eating lightly. How long does it take hydrogen sulfide to dissipate?
 

sunny

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Do a water change ASAP. It looks like your nitrates spiked up due to the glass scraping. What are the water parameters right now?
 

Wilhelm

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Nitrates are at 20 as of last night. I had done my usual 30 gallon bi-weekly change two days before the incident. Is 20 a danger level, enough to kill three healthy fish?

I guess I need to leave that back wall alone and get a couple herbivores for the tank.
 

nanoreefer22

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I agree I really doubt it was nitrates that killed your fish, especially 20ppm of nitrate.

Did you do anything else while you were scraping the glass? Did you have anything on your hands/arms prior to scraping the back wall?
 

Wilhelm

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The gloves are rinsed in fresh water, then dried and stored in the cabinet under the tank, on the cover of the evaporation top-off tank. I doubt any pollutant would have made it onto the gloves.
 

cowfish

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When you removed the algae you most likely freed up some noxious gases which temporarily "poisoned" the water, maybe by rapidly decreasing the O2 levels.

Nitrates need to be REALLY high (100ppm+) before most fish start getting affected.

If you want to continue cleaning the back glass, introduce some fresh carbon when you do and have a large (30%+) water change prepared ahead of time.
 

Wilhelm

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Curious, how long does hydrogen sulfide stay in the water. Save for the fatalities, the survivors all seem well now and began to eat normally a day ago. I plan on not going to that back wall for a while but normally replace 35 gallons at a time when changing water.

Thanks to all who offered your knowledge and experience. It is appreciated.
 
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