It irritates me that something so "natural" seems to require such a lot of intervention. Even the grandest ponds (as I noted with glee in the Sissinghurst documentary last week) can have problems with blanketweed, while a small pond like mine (8ft x 4ft) also needs a UV filter to keep it clear of algae, which means you also need a pump.
The UV works by encouraging the algae to flocculate, which basically means to clump together, so the "floc" sinks into the biological filter. This is basically a foam layer, beneath which is a tank full of plastic rings. I'm not quite sure how that bit works. Perhaps it tells the algae to floc off.
Then there's the heron deterrent, which in my case is Netfloat, a set of interlocking plastic circles. Someone once told me that a raised pond like mine wouldn't be attacked by herons because they like to wade into water, and require a long landing path. This is rubbish. I've seen a heron descend vertically onto the edge of my pond, like a Harrier jump jet coming into land on a cruising battleship.
But I digress. The current problem seems to involve both the UV filter and the pump. I suspect the bulb has gone on the UV filter, and the pump has finally been strangled by the blanketweed.
Because my UV filter has a biological filter as well, the whole thing is the size of a car battery, possibly a bit larger. It is carefully hidden from view beneath a fig tree and a cordyline. You can't see it at all, which is great - until you want to change the bulb.
At this point you have to bend double under the branches of the fig tree, and unscrew the entire top, which remains attached to the hose that leads to the pump AND the power cable. Then you have to change the bulb without a, breaking a fig tree branch, b, pulling a muscle in your back and c, saying any rude words. (OK, I'll let you off "c". I always say lots of rude words.)
Luckily, I now see that you can now get a UV filter system with the bulb housed in a separate compartment on top. And there's even a new Oase combined UV filter/pump that doesn't have to be hidden under a fig tree, but can go in the pond. Hallelujah! Perhaps a woman with a small pond and a bad back joined the design team. I think I may treat myself. I think I deserve it.