Seachem phosphorus

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The 500ml bottle dosage is 2.5ml per 80l while 250ml dosage is 0.5-1ml per 80l. Anyone knows why? Also, its potassium phosphate, any difference with lushgro powder potassium dihydrogen phosphate? Seem much cheaper than it as it treats 16000l compare to 1000l for 1kg lushgro one in terms of price ($14 vs $12).

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Btw, 2.5ml raise phosphate by 0.15 ppm according to seachem website.