Absolutely gutted to report I found Troy and one of my Synodontis brichardi trio (female I think) dead last night just before bedtime, after being alerted something was wrong when another S. brichardi was desperately trying to stay at the water surface and my Distichodus group were gasping heavily.
I can only guess it was the heatwave we've had in Southampton since just before last weekend and unusually, I stupidly didn't lower the water level on the the 6-foot tank as per normal during heatwaves, so that the filter outputs were right at the water surface to improve oxygen levels in the raised ~29C temps.
Big water change when I was about to go to bed and a half dose of emergency amounts of Seachem Safe due to temps as per instructions, everyone else looks to be OK this morning including the Euchlichthys trio, I've started up the vibration-noisy Eheim Powerline XL for more surface movement for the next few hours and I'll do another water change later.
The adage of worry when settled fish act unusually rings so true my experience.
Dreaming of a full-on 5x2x2 Zaire River rapids biotope...