get a trash can that is 25% the water volume of your tank, so if you have a 180 gallon tank as i do, you would need a trash can that will hold about 40-45 gallons.
now fill it with water, tap water or RO water,(there is only a slight difference)and cure your concrete for a week using the normal method in part 1
Part 1: which is changing the water every day or more for a week to ten days.
Part 2: after part 1 change all the water one last time and let the concrete sit in the water with no water changes and test each day with a PH meter, if your PH does not go above 8.5 after a week of no water change then it is ready for your tank.
but if it does go above 8.5 at any point change the water once and start over with part 2 again and test each day again. when the pH stays at or below 8.5 your all set.
