Trying to keep this short.
I decided to replace rwo Emperor HOB filters on my planted 75 gal. tank because of noise and because it seemed that every time I touched a filter it regurgitated more stuff into the tank than it collected.
I intstalled a new SunSun 404B (2000l/hr with 9W UV). Things went normally ok for two days, params looked like no impact on the change. I incorporatedd some biomass in the 4th tray of the cannister that was retrivedd from the emperors.
Then (i.e. two days after filter change) I made a major trim on the overgrown plants.
Tank crashed! Everything stressed, some that were smart enough gasping at the surface. Tests indicated no ammonia, nitrate, nitrite issues. I quickly instituted remedial: a water change, O2 insertion, Stability dose, circulation pump addition, ceased CO2 injection, and crossed my fingers and toes.
The distress/gasping proceeded through the day. To my amazement overnight I lost only one fish (a sailfin molly) of 40 fish in the tank. This population has happily cohabited for over two months, with the tank total span being 4 months.
All survivors are now perky, wagging their tails for food! Remedial actions are still in place, though I want to remove them today.
BIG Q: What happened? I have read of release of gas (H2S I think upon disturbing substrate. Though most of my trimming was cutting floating plants, I did dislodge and discard a large fern with copious roots pulled from the substrate.