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.
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
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.