All of the recommendations above are good for removing dust, dirt, and mineral residues from water or fertilizer, but sometimes that "dull" appearance is from the surface texture, not from a deposit.
That can happen if the irrigation solutions contain excessive dissolved minerals. Not only can they leave deposits, but they structurally affect the leaf as well. If you would like to remedy that, you're going to have to find a source of purer water, whether that be rainwater, distilled, or RO, and then feed regularly at very low concentrations, mimicking what the plants would get in nature.
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