when you say big water change how many gal should i do got 140 tank 40 gal 30 gal water change?Thanks of the info. So you are manually dosing Kalk... got it! After you do your big water change, take an Alk reading. For the next few days, be sure not dose any Kalk, don’t do any water changes and please report back with your Alkalinity readings, preferably taken at the same time every day.