Ponds R Cool

We have two ponds - 1 x 3600l and 1 x 1800l. Water travels from top (4m x2.5m) pond to other (2m x 2m) pond via a sml river, then returns via pump and underground pipe through UV filter back to the top pond. Had a green water problem which we treated successfully with barley straw extract (the water is now crystal clear) BUT there is still the algae and sludge coating all over the rocks and stones in the bottom plus the algae is attaching to the pond plants. So far bought two venturi type cleaners, one attaches to the garden hose and the second pump action type (designed for spa pools). They are incredibly slow progress; do not "vacuum" the bottom very successfully and their filters are not that fine. Most successful fix we have done so far is with a home made filter made of sponge rubber bricks at the end of the river that "catches" what you stir up before it hits the bottom pond, but looking for some better long term way to keep the rocks/stones/bottom attractive looking. Any suggestions?