If he's going for flake food, he may be trying to get protein in his diet- basically BN's are primarily vegetarian pleco's, and if you feed them too much high protein foods, it can give their stomache disorders, however, a little bit of protein in their diets once a month or so is fine. You could try putting in a cooked prawn for him and see wether he takes notice in it, if he doesn't, he's probably not looking for protein in his diet
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Does the tank have any algae in it?
Lettuce in general has next to no nutritional value, its not the best of pleco foods because of this, cucumber on the other hand is much better- when you buy the cucumber, try to go for organic so it doesn't have any pesticides or herbicides on it and make sure you give it a quick rinse.
After slicing a thick slice off it, you need to find something to weight it down as cucumber floats in water- some aquarium plants come with metal weights to weigh them down, which can also be used to weigh cucumber slices down too (this is what i use)
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Some people blanch cucumber (when you "blanch" cucumber you basically put it in some boiling water for a couple of moments to help soften it up, then put it in some cold water to cool it down) which some plecs prefer, but its mainly used for critters like ornemental snails who can be very fussy about their veg.