Hi - not technically a pond but not sure where else to post so apologies if not relevant - just tell me....

I have a water feature that includes a waterfall coming through a sleeper dropping about 50cm onto large stones and then into the liner which is about 40cm deep. About 10cm below the top of the liner is a metal grill that is used to support additional stones. This creates quite a nice overall effect.

For water pumping/filtration I have a small pump in the liner and a hozelock 4500UK filter embedded into the ground just behind the waterfall.

This all works just fine except for the stones keep getting green slime. I have just emptied the whole thing and am cleaning the stones before geting this all setup again. Any suggestions as to how I can keep the green slime away? My thoughts so far:

1 - have a fine net under the stones so I can periodically remove them and lift out the net + whatever leaves etc may have fallen in and started to decompose.
2 - have my daughter more frequently collect the leaves etc from the stones!
3 - use a chemical of some kind to help breakdown the leaves etc - I brought something from the garden centre to do this - must find that.

As there are no fish and little chance of birdlife I don't mind using chemicals etc. All ideas appreciated.

Thanks
Adam