[andestang]

Originally Posted by fkshiu

Exactly how high were your nitrates? Fish can and will survive in much higher levels of nitrates than corals can and nitrates don't usually cause sudden mass deaths.

Secondly, how exactly do you think the FX5 caused the spike?

The most common cause of "death by filter" is if it was left off for a long time which caused everything in there to die. Within the sealed confines of a canister filter like the FX5 you would have had a lot of hydrogen sulfide gas build-up. As a result, when you turn the filter back on, all the nasty H2SO3 will literally cause a monstrous fart from hell which gases everything dead in the tank. Do you detect any rotten egg smells?

Not trying to cross-examine you here, just wondering perhaps there might have been another cause.

This is all true, I have a FX5 that I run on my Koi tank during the winter. I find it gets restricted fairly easily especially if your using the water polishing pads. It pretty much quits running and needs to be disconnected and totally cleaned out (the smell will knock your socks off) If you don't your headed for trouble. You really have to pay attention to the output for any reduced flow. Really sorry about all your fish, the bad side to the hobby.