Hi, has anyone used one of these? Are they effective. We have recently moved and inherited a large pond complete with fish, frogs & newts. It has not been well maintained and there is a lot of sludge at the bottom and we have tried to treating the water but it remains murky. We are considering a vacuum to remove the sludge- do they work and is more cost effective to spend a little more or does a �19.99 vacumm work just has effectively?