Protect ponds with netting before the autumn leaves start to fall, as these are the kiss of death for your pond's ecological balance. As leaves rot into the water they release nutrients and encourage algae, as well as creating nasty sludge.
Start by pulling dead and dying foliage from aquatic and marginal plants so the pond's surface is clear. While you're at it, rake out at least a third of your oxygenating plants as they can grow rampantly left to their own devices: leave anything you remove on the side of the pond for a day or two so any creatures can crawl back into the water.
We've got pond netting in our garden centre in various different sizes: a mesh size of 12mm keeps most leaves out. If you have a large pond, stretch wires taut across the pond first to prevent the net sagging into the water. Lay the net across the top of the pond, trim to size and anchor into the ground securely.
Keep an eye on your net through the autumn and if there's a particularly heavy fall of leaves, pick them off the net's surface from time to time. You may also need to fish out any leaves that find their way underneath.