Question
I've got a puzzler: we have a fountain in our front yard that runs out of water fairly quickly when it is running. But when the pump is not running, it doesn't seem to lose water at all. The design is essentially a planter on top of a big plastic tank with the pump submerged in the tank. The water comes up from the tank and spills gently over the sides of the planter back into the tank (covered by chickenwire and loose black rocks). It has worked perfectly for more than a year until just recently.

Answer
If nothing has changed and this has worked without loosing water rapidly for a full year then only one thing can be happening. The water must be splashing out when its running or the pipe carrying water from the pump to the planter above is somehow directing water outside the fountain reservoir. Note even little drops are not so insignificant when multiplied by hundreds per hour. If all else is exactly the same that is the only plausible reason.
Does the pipe from the pump go outside the edge of the reservoir to get to the planter above? If so it could be dripping down the pipe and outside the tank below.

Good Luck

Sean J Murphy