I'll look that up!
@Simon Cole 's suggestion for food and timing. Courgette/cucumber is popular, other than that blanched green beans, sweet potato, cucumber, green pepper etc.Par boiled broccoli - I feed it to everything. Wonderful food. Guppies are less active when it's dark, and I think that might be worth a shout, although not tried it.
Which plec is it?
Feed the plec at night when the lights are out and the guppies are sleepy
Please confirm what type of Plec you have. Some graze on bogwood, for example. But, it's the biofilm and bacteria growing on surfaces which is often what they're after. I also find that Catappa leaves decompose quite quickly in my tanks. And stuff that is decomposing is doing so because bacteria are busily at work. I have Bristlenose Plecs and they graze on Catappa leaves. You will also see Plecs at work on the tank glass panels where they clean away the thin biofilm that has developed on the surface. Along with blanched vegetables, you should soon have happy Plecs.