Dead or alive?

You might've seen some fishes floating on the surface, or sitting motionless at the bottom of the RH Fish Tank... getting picked at by the shrimps or the Corydoras; dead fish.

What happened to them?

I can't tell for sure but i have a few assumptions as to their death, and a few causes i can rule out... so here goes:

What did NOT happen...
i) Puffer Fish Attack - the other fishes are well and fine, and there are no signs of fin nipping (torn fins due to puffer biting), no signs of puffer aggression.
ii) Fish Disease - the other fishes show no sign of disease till now, and the variety of fish that died means that it is not a species-related disease.
iii) Old age - the fishes could have grown much bigger than they were when they died. The Cardinal Tetras are less than a year old, can live past 2 years at least, and can grow up to twice the current length. The SAEs were relatively large, but i've seen them grow to at least double the size at death.

What could have happened...
i) Poisoning from Dwarf Puffer Carcass - the previous Dwarf Puffer died last week and we failed to remove the carcass from the tank. The fishes that nibbled at the Dwarf Puffer carcass could have been poisoned by it. Although the shrimps ate it too, i think they were immune to it, or so i assume.
ii) Nitrate/Nitrite Poisoning - because we have been adding fishes into the tank quite regularly (10 Kuhli Loaches, 10 Sparkling Gouramis, 20 NeonGreen Shrimps, 1 Dwarf Puffer) we could have added in excess of the tolerable bio load of the RH Fish Tank; amount of fish waste produced > amount of waste the plants/beneficialBacteria can absorb/use. The nitrate/nitrite level in the tank was probably already high, but when the Dwarf Puffer died last week the decomposition of the carcass caused the harmful nitrate/nitrite levels to spike (increase suddenly) and the whole ecosystem "collapsed", causing the older and not-so-sturdy individuals to succumb to the nitrate/nitrite poisoning.

Any Remedy? I have since...
i) removed the carcasses - to prevent further decomposition to add to the harmful nitrite/nitrate levels,
ii) changed 40% of the water - to remove the ammonia, nitrate, and nitrites in the water, and put in fresh/clean water, and
iii) added some special liquid - to neutralise the harmful chemicals and balance the other minerals in the water.

Hopefully there will be no more deaths among the fishes in the coming days. Should you see anything out of the ordinary, please feel free to put it up on the forum or the RH Fish Tank blog... because the caretakers of the RH Fish Tank might not be around 24hrs, and you might spot something we've missed. Thanx... and happy fish watching!

...and Happy Lunar New Year to you!