Hydrothol 191 Granular Aquatic Algaecide & Herbicide
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Product Warning The 10 lbs and 40 lbs sizes have been discontinued.
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20 Pounds - $114.99 Each When You Buy 2+

  • Details
  • Usage
  • Weeds Treated
  • Reviews
  • Fast-Acting Granular Formula
  • Great For Bottom Growing Algae & Chara
  • Control Weeds Surrounding Docks & Shorelines

Hydrothol-191 provides rapid-acting contact control for submerged weeds and algae. Hydrothol-191 even works where there is moving water such as a canal or bay area. Hydrothol-191 can be used in combination, in rotation or as an alternative to copper for control of algae. Dosage rates vary depending on what type of algae or weed is being treated and the density of the infestation.

Usage Information

From dosage rates to product labels, the USAGE tab contains all of the information you need to properly apply your pond care products.

Hydrothol 191 Granular Algaecide 20 Pound
20 Pound
Application Granular
Temperature Restriction None
Square Footage Per Container Up to 8,000 Square Feet
Shelf Life 5 Years

Dosage Rates

Treatment Area
Hydrothol-191 To Treat Algae
Hydrothol-191 To Treat Chara
Hydrothol-191 To Treat Weeds*
1,000 Square Feet 2.5 Pounds 5 Pounds 7.5 - 25 Pounds
2,000 Square Feet 5 Pounds 10 Pounds 15 - 50 Pounds
4,000 Square Feet 10 Pounds 20 Pounds 30 - 100 Pounds
8,000 Square Feet 20 Pounds 40 Pounds 60 - 200 Pounds

*Use on aquatic weeds is dependent on weed type and density.

Please note: Above are recommendations. Always read the product label.

How To Apply

Broadcast Hydrothol-191 onto the area to be treated using a granular spreader. Treat only up to half of the pond's surface and wait 10-14 days before a second treatment. In hot weather or when fish are stressed, treat only 1/4 - 1/3 of the pond at a time. Over time, the algae and weeds will begin to brown and die. Once it is completely dead, use a rake, like the Pond & Beach Rake, to remove dead algae away to prevent an accumulation of dead algae and muck.

When To Apply

Hydrothol-191 should be applied when algae and weeds are actively growing. Begin applications when algae and weeds first appear. Unless steps are taken to reduce the overall nutrients in the pond, multiple applications may be needed for full season algae control.


  • Do not use in ponds with koi, grass carp or goldfish. Not for use in water garden or koi ponds.
  • Wait 7-25 days before using treated water for human drinking water.
  • Wait 7-25 days before using treated water for livestock and domestic animal consumption.
  • Wait 7 days before using treated water for irrigation on nursery or greenhouse crops.


Weeds Treated

The first step towards effectively treating any pond weed is proper identification. Hydrothol 191 Granular Aquatic Algaecide & Herbicide is recommended for use against each of the weeds listed below. Click on your desired weed for more information and alternate control options.

  • American Pondweed
  • Chara
  • Coontail
  • Curly Leaf Pondweed
  • Filamentous Algae
  • Hydrilla/Elodea
  • Planktonic Algae
  • Southern Naiad

What others are saying

5 out of 5

Customer Rating

"I was surprised"Kate Ludtke of rogers city, MI

My weeds were outta control and i called in to see what I could get, they said this was good. I recieved it in 2 days amazingly, and used it right away, took about an hour for me to figure out how to open the top, but once done it was easy to apply and killed most the stuff dead in like 4 days. I'll be buying more soon.

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5 out of 5

Customer Rating

"Cleaner shoreline!"Henry of Sterling, MI

I was having trouble with pondweeds on my shoreline. The Hydrothol helped to kill those weeds and made my shoreline more attractive. I don't mind stepping into the water now.

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3 out of 5

Customer Rating

"Hydrothol 191"Alan Blanford of Potosi, MO

I have about 1/2 acre pond. Used 2 10 lb. pails. Thought it would do the job, but might have to to another round to do a better job.

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