Oh,
@Matt - I'm so sorry to read about your agassizi male; how horrible for you.
Did you find out how the others in the store are doing?
There have been some fatalities, a few at a time dotted over the course of the past few weeks, but generally nothing unusual and no definite signs of illness that have required them to be taken off sale. The few that remain looked active and healthy. (Having said that, it was only later on when I re-examined each of my own fish in detail that I noticed the missing portions of tails in two of them, something I hadn't considered checking on those in store.)
Similarly to you, I'm beginning to wonder whether I'm responsible not only for today's event but also for the missing portions of tails. I thought we still had an old glass tank, stowed away, as a temporary back-up tank for the main tank in the event of an unspeakable crisis and also for the purpose of quarantining; it was only a couple of days beforehand that I discovered we no longer had it (I must have given permission, perhaps while otherwise distracted, for it to be disposed of) and realised that I'd have to dual-purpose the hospital tank as the quarantine tank. This tank is only 17 litres and a bow-shaped front - basically, just like this
https://www.ebay.co.uk/itm/Disney-Frozen-Aquarium-Kit-Boxed-Tested-Trusted-Ebay-Shop-/272840145831 - which I thought would probably be alright for the short-term duration of quarantining. Now, however, I'm beginning to think this was a mistake - the fish have been extremely and almost constantly active (like Duracell bunnies) and perhaps this hasn't been fair keeping them in such a small environment. I'm wondering if some chasing I've seen in the past week has led to the missing portions of fins on two of the fish, while today's bid for freedom (or whatever it was) was a consequence of this... Needless to say, I've since been doing some initial research into a new quarantine/back-up tank.
I'm still trying to establish how the fish did escape. The tank lid was not removed between the time I had been examining them in detail and this little fish met its end on the carpet. Therefore, either it catapulted/kamikazed out of the rear cable holes (potentially hitting the wall behind the tank and then bouncing or plummeting forward onto and off the chest of drawers on which the tank was located to ultimately land on the floor in front of the tank) or else it kamikazed out through the 0.5x1.0cm hole in the fish auto-feeder which I'd been experimenting with the past couple of days and which was located towards the front (I'd removed the flap and set the auto-feeder on top of this where it had fitted perfectly with no gaps but had had to keep the lid of the auto-feeder open as the flakes were clogging up). The latter seems more likely as this would more likely explain how the fish ended up on the floor in front of the tank. Whether it was making a bid for freedom because it found its environment too small, got a fright (although I don't think that can have been possible during the timeframe it occurred in), was attempting to retrieve the clogged-up food from the auto-feeder (plausible as some of these new fish have almost been leaping out of the water before the food hits it when I've been feeding them), or had some sort of epileptic seizure / heart attack / stroke and just happened to fly out through the hole, I'll never know.
Separately from this, in the main tank, the newer of my two nerites seems to have developed an obsession with pushing itself behind the main filter and causing it to float out from the wall. I can see this potentially causing a problem with some tank inhabitants ending up behind there if this isn't sorted ASAP, and so I definitely won't be moving the newbies into the main tank until this is resolved. Filter wool isn't an option - the downside of a corner filter perhaps, that it floats out at an angle - but I need something to help the bottom of this corner filter attach itself to the wall of the tank better, so I'm exploring the possibility of magnets inside the filter and on the back of the tank wall to enable this (although this is turning out to be a minefield).
Exhausted - just hope I sleep tonight and have no fish-themed nightmares!