How did you even find it? Little stuff like that drives you crazy til you track it down. Four bucks beats throwing parts at it.

That line about the green water got me. I worked loss prevention at a dying mall in Ohio back in 2009 and we had the opposite problem, they cut the fountain first and the whole center court smelled like a fish tank by week two. What struck me in your post is the filter guy. Somebody is still paying a human wage, buying filters, treating that water. That is not a rounding error. That is a line item with a name next to it on a budget sheet. Malls do not keep dead water alive for buyers, they do it because the moment that pump stops, the place stops being a mall and becomes a warehouse, and warehouses sell for way less per square foot. The fountain is basically a legal fiction with a motor.