People have use clove oil to sedate fish. You should be able to Google some info on that. However, clove oil can also kill, so you need to get the dose right.
For my goldies, I use salt. At 0.9%, goldies gets very sedated after just a few minutes and are easy to handle. I don't know if that would work for your parrot fish.
To remove gravel from the gills, you'll likely need a real anesthetic. I doubt if a fish would let you get at the gills even when sedated (you can try, I suppose). For real anesthesia, you need a vet (or at least some medical type that can get a hold of anesthetics .... and some experience in ventilating fish .... just saw a science show where they use a powerhead to force water through the mouth & pass the gills to keep the fish "breathing" while anesthetized.)
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80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.
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