Double check me...as I have already proven one flub up today.
But 16 oz = 1 lb
Assuming we are using 4 oz/gallon MAX. And assuming you have 0 now.
140 gallons of water.

4 oz * 140 gallons = 560 oz. 560 divided by 16 oz = 35 lbs MAX. I would start off with much less.
Like maybe 5 lbs. Mix lightly with lots of water and slowly introduce to system.
I weighed mine out by the lb, and dosed at 1 handfull every 30 minutes in a 325 gallon tank with heavy aeriation. Maybe there's a better way to do it...I don't know. So don't crucify me....this is just what I did.