Thanks. I began to think something like this was happening when we investigated the filter box yesterday and found it so gunged up. We cleaned it, and it might be just my imagination, but I think it might be a bit clearer this morning. The filter in the past has needed very little attention. We've inspected it once every couple of weeks, but it hasn't usually needed any attention at all. It's been like this for nine years or so. The sponge filter on the other hand has needed very frequent cleaning. Weekly, or more in the summer.

Obviously the pump cage has changed the system somewhat. So the so called "lower maintenance" that the pump cage offers merely swaps cleaning out the sponge filter frequently for cleaning out the filter box frequently. Well, there you go!

I think we'll persevere with it a bit longer and see what happens. See if it is a better system. The main advantage I can see is that you don't release half of the muck back into the pond when you try to remove it for cleaning, as you do with the sponge filter. I think there's a bit of a backlog of gunge from when the old pump was on a go-slow without our realising, see what happens when that clears.

Thanks again.