All fish died overnight

SnailM

So...here is my sad story.
This morning I woke up to a 55 gallon tank full of dead fish.

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.

Brandonl87

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

fish monger

Any kind of chemicals sprayed in the general area ?

Pearl2011

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

SnailM

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!

Brandonl87

Just seems weird a few degrees would do that though. But i am sorry bout your fish :(.
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Pearl2011

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.

Tigris

It could have been from the increase in CO2, there is such thing as too much.
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katschamne

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.

Mikaila31

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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