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I am working on a pond right now...it is about 700 gallons. I have noticed that it is very cloudy and green. At Lowes, they have Algicide for relatively cheap as well as a Barley pond clarifier (as well as a liquid clarifier). Would these be safe to put in the pond once my RES is in it?

Most algicides say whether they are toxic or not.
But, truthfully while it may reduce or get rid of the algae once unless you fix the underlying cause it's going to be an uphill battle.

I tend to stay away from chemicals. If you add some aquatic plants that spread to cover the surface of the water and block the light (water hyacinth and duckweed are really good in this respect), it would at least help cut down on algae growth. I know of some people who use barley (safe) but have never tried it myself.

I have used Algeacide in my ponds. It works but as has been said the algea will just come back. The algea wont go away completely as well. It is great to kill string algea but "pea soup" algea wont do so well.

Having lots of shade on your pond and a chemical balanced pond will go a long ways in clarity. If all you have is "pea-soup" algea a UV bulb will kill all of that stuff. A filter would be a good investment as well but the UV will kill the algea.