Be Careful When Using Bluestone For Pond Algae Control

It’s not as easy as the Internet would have you believe — particularly if you have fish in your pond

Doug Green
Nov 5 · 4 min read

You’ve probably heard somebody say they used bluestone for getting rid of algae in ponds or you’ve read somewhere about this miraculous method. Let’s put our enthusiasm aside for a few brief moments and consider the reality of what happens when you add this to water.

Photo by Alessio Fiorentino on Unsplash

Bluestone is copper sulfate and it does indeed kill off the blue-green algae in water if it applied at concentrations high enough to do the job. It is sometimes recommended for commercial fish production as well as keeping water clear for animal drinking in agricultural settings. I’ve experimented with this material in my own ponds and I have some thoughts for you as well as the raw data you’ll require if you want to o similar experiments.

First Thing to Understand

To begin with, if you’re growing fish in this pond be aware that concentrations high enough to kill off algae are almost high enough to kill fish. And it isn’t just the amount of bluestone you add that does the trick, it is the total alkalinity measurement of the water when combined with the bluestone that produces an algae kill and possibly a fish kill.

In other words, the higher the total alkalinity, the more you can use and the closer you can come to lethal doses more quickly.

Algae is killed at one-half part per million while fish are killed at 2 parts/million.

This amount is much more easily managed on a large pond than a small pond.

  • On commercial sized ponds measure in acres, we’re talking using pounds of material to achieve these concentrations.
  • In small home scale ponds, we’re talking the difference between a half tablespoon and a three-quarter tablespoon or 2 grams and 4 grams.

Bottom Line

If you’re growing fish in a small home scale pond, then the chances of overdosing with bluestone are high.

There is simply too much variability in the water quality and the fish to recommend using this material.

Do not consider using this material on small home ponds

Fish Sensitivity To Bluestone

Also, if you’re growing any of these fish in your pond, understand these fish are more sensitive to bluestone than others:

  • Carp,
  • Grass carp,
  • Goldfish,
  • Koi,
  • Trout and
  • Salmon.

Having said that

If you don’t have fish in your pond, and if you’re determined to play around, here’s the data you need to know.

In order for bluestone to work effectively, the concentration of bluestone must be equally distributed throughout the entire pond water.

You’ll require some form of water movement — be it a pump, filtration or aeration system to get the water mixed up.

Basic Math

To determine how much to use, you’ll need to do some basic math.

The first is to calculate the size of your pond. You need to know the “acre/feet” of your pond which is a measure of the total cubic area of water. To calculate this — multiply the surface area of your pond by the measure of the deepest part of the pond. If your pond has 5000 square feet then it is roughly 5000 divided by 43,560 or roughly one-eighth of an acre. Multiply this area by the depth 12 feet to obtain your acre/feet measure.

Because you have sides of the pond and shallow areas, it is often recommended you multiply this result by 40–50% to account for the variation in depths. If your pond has straight sides, then go with a higher number.

Obviously this is somewhat of a judgment call (again, a reason why smaller ponds are more problematic when it comes to using bluestone).

Knowing the acre/feet of your pond, you now need to know the total alkalinity of the pond.

Go to a pool supply store and get the water tested. Note home scale testing kits are not accurate enough for a small pond.

Total Alkalinity

  • Below 20 Copper sulfate should not be used
  • 20 mg/l use .6 lbs/acre-foot
  • 50 mg/l use 1.3 lbs/acre-foot
  • 100 mg/l use 2.0 lbs/acre-foot
  • 150 mg/l use 2.8 lbs/acre-foot
  • 200 mg/l use 5.5 lbs/acre-foot
  • Above 200 — do not use as the effectiveness decreases with increased alkalinity.

There are household conversion rates available on the Internet but these are mostly measured in gram differences. The practical value of these for small ponds is minimal as only a slight miscalculation of the depth or total water volume of your pond will result in a fish kill.

Does This Sound As If I’m Enthusiastic About Bluestone For Algae Control?

Good — because I’m not.

Doug Green

Written by

Former nurseryman, now writer and curious about what’s over the next hill and how to get there in either my Triumph Spitfire or sailboat.

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