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There are several variables like PH value, calcium and magnesium values etc. It's not easy to control all these manually. You can get rid of chlorine in water by keeping it for 48 hours before adding to tank but not other things. That's where Seachem Discus Buffer (Here) comes to play. This can replicate the ideal Discus environment by lowering pH & lowering General Hardness. Also it softens water by precipitating calcium and magnesium. This can help you to keep your Discus fish safe. You don't want to change water and night and then in morning look at Discus graveyard feeling like a mass murderer.
But remember Discus Buffer is for lowering pH and GH but if you want to get rid of chlorine, chloramine, or ammonia then you need to use Seachem Neutral Regulator (Here) along with it. The Neutral Regulator conditions the water, rids it of chloramine & ammonia and also helps Discus Buffer to keep to pH value at 7 or low.