It sounds like you will need about 200# of sand. When you add it to the filter, make sure to swing the diffuser out of the way so as to not get sand into it. Since it is difficult to determine exactly how much sand is needed, you should probably plan on two trips to a supplier, either buy too much and return the extra (or keep it dry and add some in a year or two) or go get what you might need and go get the rest later if it is not enough. Either way, since you have a filter with it's own fill port adding sand is relatively easy and sand is not all that expensive.

You MUST use sand that is designed for swimming pool use. DO NOT substitute any other or you will end up removing all of the sand in the filter and starting over. And, you will have 600+# of wet sand to dispose. Not an easy task.