Came back from work, one of the WCMM is in a state, clearly dying. Removed it and dispatched it. Any ideas what's happened? No other fish showing any signs of distress. It was 18 months in the tank. Cause for concern or just one of them things?
Very likely just one of those things - fish may suffer tumours & misc. cancers as well as various states of infection (bacterial & viral) that can cause "swelling" (also "eggbound" if female) so I'd not dwell on it ... OTOH anytime that I notice fish feeling a bit under the weather etc, I increase water change frequency, doing a large water change the same day/night if possible.
In general our tanks are always a bit more "polluted" than any wild habitat, also greater fish density & a mix of fishes confined to a small area whose behaviours may be stressful ... so water change is always a great start to sorting any fish health issues.
Observed behaviour in video is pretty typical for fish in (most likely terminal) distress
Tumour would be my guess as well.fish may suffer tumours & misc. cancers as well as various states of infection