400-450 gallons, when I was in the 190 I used one scoop.

It isn't always the volume of water, but also your demand that dictates how much you dose. If I didn't have any sps or lps I would probably just rely on water changes. But because I've got a bit of sps in there I found increasing it to two scoops daily really helped the sps grow. I also do a 15-18 gallon water change each weekend.