Just wanted to say I've managed to rehome the Common Pleco and the Clown Loach. The swords that were unhealthy in the 40l were on the mend, but the tank they were in didn't seem to help them, so after several weeks, I moved them one-by-one into the 230l tank. Since moving them, they've all recovered.
The female platy that was being chased by the male black ruby barb finally broke down from the stress of it all and developed pop-eye. So I moved her into the 40l tank to quarantine her from the others. After two weeks of intensive trying, I decided it was time to euthanise her. I wish I had had the time to do so as 2 days after I was going to do it, she died. She's now gone to the toilet up in the sky.
I am going to totally take down and sterilise the 40l, perhaps using it as a QT tank in the future. For now, my big tank is doing well and I look forward to not having to siphon the pleco's waste every other day!
Here's my stocking now in the 230l
Off to ask for advice for "show" fish as the Pleco was my son's favourite and now I'm afraid he'll lose interest.