Keep offering it. He will find it and then he will be a happy camper. Mine likes zucchini better than cucumber. I feed veggies 2-3 times a week, along with frozen food. I have a heavily planted tank and my BN's don't bother my plants except to clean them.
I slice up a veggie then freeze the slices in a ziploc (separate so they don't stick together). Then I take out a few and let them sit in a cup of warm water for a few minutes then feed. They are a nice soft consistency for the fish at that point. (My other fish like them, too).
If you get on Amazon and search for veggie clips, there are two kinds. One is an orange clown fish--those break really easily. I've gone through several that didn't last long. The clear ones (come in a pack of two) have been awesome. Still on my first set of those.
Also, I wouldn't leave the cuc/zucchini slices in there for more than 24 hours.
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