My pond has been going just fine for a couple years now but lately, I have
seen a decrease in water flow.

When pumps fail, is it a "working" / "not working" scenario or is it a water
flow just slowly starts going down? For some reason, I thought pumps kind
of worked or they didn't. If they failed, they didn't work at all.

Couple notes on the pond:
* Whole thing is about 1000 gallons - 2 ponds - 1 small one flowing down two
35' streams into a lower pond that is about 5' x 12' x 2'. Extenal pump
(forgot name) that goes up 2" PVC - ~6' drop over the 35'.
* the filter is a notched PVC piping system with screen door screen and a
few inches of pebbles (Moorehaven folks in Seattle pointed me in that
direction).
* We have about 5 or 6 gold fish in there for the past year or so. The
water has been very clear but the bottom of the pond has more "gunk" on it -
my guess is due to the fish. Hopefully the extra floating plants and
lillies and grasses I have in there will start cleaning up the stuff more.
* I seem to have more string algae this year (probably fish again).
Hopefully the barely bales will start getting that under control.

Even when I clear off the bottom gunk (which doesn't take too long), the
flow doesn't really increase. Could it be that the gunk has filtered down
through the rocks \ screen and plugged the pvc filter?

I don't want to replace the pump if that isn't the culprit. On the other
hand, I don't want to drain the whole pond and replace all the rocks just to
find out that it was the pump the whole time.

Opinions \ advice is appreciated.

Thanks
Darren