The "device" that you're probably thinking of is a "magnet"....an inline magnet which is supposed to ruin the structure of algea.
They don't work
Instead, get a decent UV....10 watts of UV per 1,000 gallons of pond .
If you are getting a problem with blanket weed.....i (and many other koi keepers) use cloverleaf blaketweed cure....it is superb stuff although it costs �15 ish.
Failing that, getting the balance right between water, fish, aquatic plants and marginal plants is a good start