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I've honestly never had any issues switching between food for my fish. Try not feeding for a few days then offering the pellets. Flakes are alright but I wouldn't use them as a staple.
NLS makes good flake food. I use NLS Optimum Community Flakes (with garlic) for my tetras. For my rams, I incorporate NLS Thera-A pellets (1 mm). My four-month-old GBRs take both readily.