Yes sirre Phillbo, gallonage is a word (but I'm not sure sirre is):

Main Entry:
gal•lon•age
Pronunciation:
\ˈga-lə-nij\
Function:
noun
Date:
circa 1909
: amount in gallons

Waste, she doesn't even know the filter type at this point, but if it's a sand filter, I imagine it will just involve a lot of backwashing when 12-15 above baseline psi. If it's anything else I'll come back with questions as I know nothing about other kinds of systems. Thanks for the input re: how much bleach to add. Will report back at some point (could be weeks away).