You'll find a wide range of how much per gal that is suggested. On average 2 TBS per gal is about right, but personally I don't really measure when I do it. I just use about a handful for every 10g when I do it. Also best to do it slowly over 24hrs so the fish can adjust. So rather than add the full amount for say a 75g. Just add enough for about 25g, then 10hrs later the same amount, then 10hrs after that the same amount. That's just the way I do it and have always worked well for me, I'm sure others will probably have other suggestions as well.