To remove the plec's food after 3 hours is deeply, profoundly erroneous advice. It needs to soften up a while before the fish will even have a sniff, let alone eat it. Possibly someone has heard something about raw vegetables making the water cloudy by going all mushy (which is true, to a certain extent), and tranmogrified that into the idea you need to pull the food out.
Anyway, I've not had much luck with potato, but my royal panaque has eaten all sorts of other things over the 10+ years I've had her. Courgette (zucchini) and blanched lettuce are favourites, but also eaten is the odd piece of soft fruit (e.g., slices of peach), cucumber, and carrot. Other people use broccoli, frozen peas, spinach, and all sorts of other green veg. Don't put too much in at once. For my 7-inch Panaque, I'll put in an inch-thick slice of courgette, and that'll happily last her a couple of days at least.
Above all, she eats wood. Panaques must have access to bogwood, and I personally think it is worthwhile making sure all plecs can get some if they want it. If nothing else, it is good 'fibre', but some species do seem to be able to digest it (e.g., see
this).
Also remember many plecs feed primarily at night. There's not a lot of point of putting the food in during the day for these species, though other plecs are more active by day. Don't forget to give the plec meaty foods, too. Frozen prawns are ideal. In the wild they eat all sorts of insect larvae and worms, not to mention carrion, so they aren't just herbivores.
Cheers,
Neale
I put the potato in there in the daytime but read that you had to remove it within 3 hours, so I did that but he hadn't eaten it by then!
Is the potatoe still there the next day, if so, maybe you should try a different type of food, maybe wafers? 
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