First time pond owner & fish mate pump

Greetings
Just discovered this site and as i'm owning my first house with a garden
i think it may be invaluable.

Anyway i have bought a house with a pond, not a big pond. It has a fish
mate 2000 fountain and just discovered the pump round the back (a fish
mate 2500UV) which feeds a mini waterfall.

I've just emptied out the pond as it was filthy, full of leaves and muck
and thought i better find out what this equipment actually does. Now as
i say i'm a complete novice with ponds so please be patient!

What does the pump do (apart from feeding a flow of water to the mini
waterfall) does it clean the water? is it just for keeping fish. We've
not had the pump on since moving in would this have stopped the pond
becoming so mucky.
I don't know if i should open up the pump and replace or clean anything
before i fill the pond for summer.

Appreciate any guidance on this!
phil




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