Thanks for the ideas. As the tanks are integral fiberglass with lids that are not very thick, any form of a float or pressure type gauge may be difficult. Though, the lids will be coming off for a re-seal, so it is a good time to look closely at this.
OW, it seems a simple clicker flow meter would be fine. The Grainger option states: Not for Use with Water Applications. It seems there must be a simple (and cheap?) water flow meter with a total amount passed out there.
On to the search.