The house I bought back in '80 came with a pond. Indeed, ponds were something of a neighborhood craze fifty or more years ago, as a dozen nearby houses had 'em. I kept it as a place for toads to breed...as toads eat slugs, so I'm gonna help 'em out. Fish were there to eat the skeeter larvae. (Chytrid fungus has wiped out the toads in the ecosystem, but that's another story....)

The pond has two filters, a hidden biological one that feeds an antique pitcher pump, and a bottom-of-the-pond box filter with a new magnetic-drive pump inside. This latter one has learned a new 'trick' I simply cannot comprehend: it tries to float. After a few days of use, it surfaces like a submarine and the pump will move it about. Sure, I could put a few rocks in the bottom, but I don't see how the porous filter fabric top can retain air to create positive bouyancy - especially when the fluid flow is "in" not out.... How can something submerged with a porous 'roof' float?

Cheers