If I have about 185 gal fish tank [bottom portion of IBC], and am starting with two 75 gal grow beds (sans rock) [top 12 inches of IBC].
Is there an equation to determine the correct size (how many gallons) my sump should be?
I have on hand what should be a good 40 to 50 gallon basin I would like to use, but is it big enough?
Rough rule of thumb is you need the sump to be big enough to hold all the water to flood all the beds that will be flooded at one time plus a little bit extra to keep the pump from running dry and enough to handle not having to top up every day (or use an automatic top up valve.)
So the estimation of how much water it takes to flood a bed is usually about 40% of the bed's total volume. I usually use 50% just to add in an extra margin of water in the sump.
So if you will be flooding two 75 gallon beds at one time, you probably need at least one 75 gallon sump tank and I would say you might be better off with 80-100 gallons of sump tank to play it safe.