Well.. something came in the night. Snatched my pleco and captain harlock.
Half a dead platy was sat on the filter scumming up everything.
A waterchange and some prime and everything was good.
The hose got knocked over the edge a few hours later.
All the water drained except 4".
I filled it again.
My water degasses from 8.4 to 7.5 in about an hour.
So the pH swing can be deadly.
The next day one of my ryukins was floating.
All is well now and I have two new goldies.
A black moore and a calico ryukin.
Also.. This month the tub pond is being torn down!
To be replaced by a slightly larger (I think around 200 gallons) more attractive feature.