This is what happened.
I don't have a water testing kit - when I've had concerns in the past I've taken a sample to my
LFS & they test it for me. It was late when I spotted the problem so the
LFS was shut. I wondered if the pump wasn't working properly because the water was cloudy. I took the pump apart, gave it a good clean and stuck it back together again. There wasn't much else I could do so I went to bed worried about what I'd find in the morning.
Morning came, two dead fish (out of 14), one angel and one neon tetra. Sad but not the fish soup I had been dreading. All other fish fine and all back to normal. Lessons for me are to get a water testing kit and investigate what remedies I could keep on standby for the future.
In answer to your questions:
Filter refresh is not a complete replacement of a cartridge. My tank has a filter tower with (from bottom up) fine blue filter, 2 coarse blue filters, 1 black (carbon) filter and 1 white (cotton type) filter. The white gets replaced weekly, the black 5/6 week intervals & the blues have longer intervals, up to 6 mos for the fine one. There's no 'bio wheel' but a pump that sits at the top of the filter stack drawing water up through the filters and cycling it back into the tank.
I've had the tank and most of the fish nearly two years. I've always used rainwater + standard conditioner and never had a problem like this before.
Thanks for your input everyone.