Several months ago (as best as I remember now) I lost a Gold Nugget, a Yoyo and 4 angels (one of which was my 5"-6" black angel which I had raised from a juvie) due to, what I believe was, an off-breed fertilizer.
A portion of this was odd in that Queen Pleco's are very, very sensitive fish and they did not seem to be affected. Now to the topic at hand.
This past Wednesday I awoke at midnight "played on my computer" while "enjoying my fishies" via room lighting only and went back to bed at 1 a.m.
I turned on the aquarium lights at approximately 6:00 a.m. and disaster!!!
Two Yoyo's and one Queen were dead;
Only one sterbai in sight and it would not move when I tapped the side of the aquarium that it was nearest to;
The remainder of the Yoyo' were laying still on the bottom with their lips and tails turning red;
One gold nugget was hanging on the back of the tank which he has not done since he was a juvie;
the SAE's were sedentary on the bottom but
none of the other fish seem to be affected by "whatever it was".
All! water parameters were normal!
Removed the fish which I "knew were dead";
Did a 30% WC with dechlorinated tap water which I induced into the sump in order to not cause an instantaneous change in the water temperature;
"Sucked most of the water out of the sump" in order to allow replacement with RO water.
I unfortunately did not "think of" activated carbon filtration until that afternoon (I presume that this was because I am "so adverse" to chemical filtration).
I had two "big jugs" of Black Diamond activated carbon in the garage.
I placed the carbon in one of the large jug into a media bag and the media bag into my sump.
At this "point in time" I am still removing tank water from my sump and replacing with RO water and continued to do so throughout the night.
I also thought about H2S and CH4 gases even though no odors were perceptible emanating from the surface of the tank water.
I pythoned out the surface and first layer of substrate as best I could (ie. working around the plants and obviously not being able to get underneath the rocks and wood).
The next morning when I turned on the aquarium lights one GN was dead.
I cannot remember what "else I did" but there were several other actions.
1) have any other ideas as to why this meltdown may have occurred or
2) have had experience in this type situation and implemented remediation other than that which I have set forth.