You could make a simple water gauge from a plastic float and a flag to reveal the water level in this container.
Reader T .J. Byers described in the article Water Level Monitor how he
rigged up an electrical circuit between his water tank and
his home so he'd know—at a glance—just how much
water he had in his storage tank.
Well, Noel Stone of Powell River, British Columbia has come
up with a different homemade water gauge ... one that
is not as exact as Mr. Byers' invention, but sure does let
the Canadian know when his cistern's starting to run low.
Noel constructed a simple "fluid flag" by tying an empty
plastic bleach bottle and a small red banner to opposite
ends of a long pole. Then he put a large spike through the
stick's center (to provide a rotation point for his device)
and attached that nail to the inside of his holding tank.
That way, whenever the water level of the storage facility
drops, the float bottle descends and the flag rises ... until—eventually—the signal pokes out the top
of the tank, telling Noel that he'd better start
cutting back on his water use!