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Discussion Starter #1
So...here is my sad story.
This morning I woke up to a 55 gallon tank full of dead fish. :cry:

I would appreciate anyone's help in trying to find the root cause, so that I can avoid making the same mistake again in the future.

Tank parameters:
55 gallon, live plants, sand substrate, established (since Dec. 2011), added CO2 from tank,
Filtration: Biological, Chemical and Mechanical provided by 2 canister filters: Rena 2 and Fluval 406
pH 6.4, Ammonia 0, Nitrites 0, Nitrates 40-80 ppm
Copper 0, Phosphates were a bit high (I forgot the number), but nothing out of the usual
I didn't have a chance to check for Iron or KH/GH yet, but I'm planning on doing that tonight

Fish:
School of Neon Tetras - about 25
2 Siamese Algae Eaters
2 Cory Cats
1 Upside-Down Cat
1 Zebra Danio
15 Paradise Gouramis (more than half was not adult size yet)
10 Apple Snails
3 Nerite Snails

The snails made it out alive and I saved one cory cat, but I'm afraid it will die too, because it looked very lethargic this morning.
Fish had no injuries, no spots, no faded color or rapid breathing and were swimming happily around the tank yesterday night.

Here are a few things that have changed over the past few months/days (I'm sorry if some of them seem ridiculous, but I'm grasping at straws right now):
- 3 weeks ago introduction of new Fluval external filter (but old filter is still installed and run side-by-side until new one is cycled)
- yesterday morning I noticed that my CO2 bubble count was a bit low, so I increased by a bit, but I have two air stones bubbling all night
- sudden change in pH due to increased CO2?
- 1 day ago I opened new jar of flake food (which was also fed to fish in other tanks, they live)
- Temperature change???? It was a bit of a chilly night and I had my heater set low because of recent hot days.

Any comments/ideas/constructive criticism is welcomed.
 

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Hey maybe completely wrong here and or seems stupid but the new filter you put on a few weeks ago what do you have in it? like Bio stars? Chem zorb bags?. I know you have to fully wash those "bags" out if not could kill the fish maybe?. Just shooting out ideas is all
 

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Hmmm, nitrates are a bit high, but that seems to be the only problem. Do you know how much the temp changed?
 

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Discussion Starter #5
Thank you all for sharing your thoughts!

BrandonI87 - The new filter does have those ceramic balls or stars (or whatever shape they are) as well as activated carbon and a coarse and fine sponge filter. I rinsed all out in cold water as recommended by the manufacturer. That being said, there may have been leftover particles/toxins in it. However, I would expect an adverse reaction one or two days after installation, not three plus weeks.

fish monger - Nothing was sprayed in the area. The tank is covered, too.

Pearl2011 - I'm actually starting to think it might have been the temperature. I didn't read temp last night, but this morning it was about 24C/75F. I was indeed a chilly night, I suspect the temperature might have dropped by about 5 degrees at least. That would explain why the snails were ok. I read that applesnails are very hardy.

Thank you all!
 

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Just seems weird a few degrees would do that though. But i am sorry bout your fish :(.
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5 degreed C or F? If it was F I would be surprised you didnt have any survivors, C I understand that. My bettas survived 7-8C drop in november, but that surprised me too.
 

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I would take my guess with sudden change in pH due to increased CO2 and with that the elevated levels of nitrates. With the drop in pH it also altered the alkalinity making it more acidic.
 

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What was the Ph change you experienced? I would also suspect CO2 especially since your water looks soft. Do you use a drop checker? Bubble count really doesn't mean much for judging CO2 levels. Though the air stones should of prevented any poisoning unless you really cranked it.
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hmmm....i have hgeard of some heaters(i forgot which brand) working fine for months, but then automatically going to 100+ degrees, then switching back in a couple of hours. what brand do you have?
 

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Oh I am so sorry you have experienced this!!
I have a 75 and twice have had heater failure... changing from 78 to 67-68. No fish loss at all. No Ich outbreaks from stress. I don't think it would be the temp change. Cause even if it fails completely, the temp will slowly drop to room temperature... it's not like you dumped a bucket of ice into it which would be a shock for sure. If you do a water change with the wrong water temp or forget the de-chlor, death is noticeable very fast!!
No new ornaments? Pieces of wood or rock? A new plant with metal on it?
What about an electrical shock of some kind? Do you feel any tingling when you touch the tank?
 

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Agreed any heater can malfunction. I have never had a marineland heater but I have still had two different brands malfunction on me. Submersibles IMO are one of the shortest lived types of eqipment.
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Discussion Starter #18
Thank you all for the brainstorming!

Here are some answers:

Pearl2011 - I was speaking of degrees F. I still haven't crossed this off the list of possible culprits, but I've been monitoring the temperature of the tank after the disaster and I don't see drastic changes. It seemed unlikely to me that about 50 gallons of water would cool down this quickly.

Assault0137 - I have a Aqueon submersible heater TA200 model. I felt no tingling when I stuck my hand into the water in the morning to retrieve the fish. The heater is still in the tank and seems to do its job. I'll do some research on it to see if anyone had problems with this brand. Thanks for the tip!

Jakiebabie - No new ornaments, drift wood or plants. Also, no water change the previous night.

Yet, all of you who brought up CO2 and pH may be onto something. I do not have a drop checker (I will make sure I'll get that). So when I checked the pH in the morning it was around 6.4, which is as low as the kit will indicate. It might have been quite a bit lower in fact. That is not what it usually is. I believe I've kept my pH at around 6.8 before.
Anyway, I found that the CO2 tank is empty. It must have run out, maybe the night of the disaster. It actually filled with water, meaning that whatever leftover gas there was must have been pushed into the tank. You may wonder why I didn't install a anti-backflow valve. I do too. Hindsight is always 20/20. That and the lower pH makes a lot more sense to me than a sudden temperature drop.

Thank you all for helping me with this. I still have a lot to learn.
 
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