Having all these mystery issues and a sick fish in my synodontis tank makes me want to be more prepared when my plecos fall ill. As far as I know, the plecos are doing wonderfully right now...but so were the synodontis before some mystery factor came into play. So to be clear, I don't think I have any sick plecos, just want to be more prepared. Good water didn't prevent my synodontis from getting sick so not counting on it to prevent my plecos from getting sick sometime either.
It got me thinking, what are the typical signs of a sick pleco?. I realized, more so then the synodontis, I think they could get sick un-noticed and die before I could treat them due to their normal behavior of hiding. Someone mentioned they stop hiding as a symptom of illness. If so, that would be a useful clue.
- if the pleco doesn´t hide anymore (except species which don´t hide that much in general, like Otocinclus e.g.) - if it behaves apathetic - if it breathes faster than usual - if the belly is sunken in - if the excrement is white
Those are some of the typical signs, when noticed you should pay more attention.
I agree with Acanthicus, but beyond those generic signs (and they are sometimes difficult to spot or differentiate from "normal"), it gets very difficult. Your question is a bit like saying to a car mechanic "My old car broke down last week, how do I know when it's about to happen with my new car?" which I'm sure you understand would either be answered with a fairly generic answer, or something sarcastic or not at all. Basically, any change (especially sudden changes) in behaviour should be watched carefully. And just like with cars will last longer with regular maintenance and using good quality spares, looking after your tank with good fresh water, etc. will help a lot in keeping your plecos hearly.
I have to second Mats advice. I don't recommend randomly removing fish for health checks since just that bit of stress could allow an opening for disease. Rather, maintain good water quality and food while watching for changes in behavior or condition.
Since improper food can also be a cause of intestinal problems, I would also add bloating to the list Acanthicus posted.
Impossible only means that somebody hasn't done it correctly yet.