Garden Endeavors


(Aluminum Sulfate Al2(SO4))

If you have problems with excess algae and turbidity, alum may be just the thing you need to achieve the crystal clear water you desire.

Pay close attention, however, to the following caveats:

  1. Alum contains aluminum, which is toxic to fish in acid water. You MUST have achieved well buffered water with a carbonate hardness greater than 90 parts per million and general hardness above 150 parts per million, and your PH must be at least 7.2, BEFORE using alum.
  2. Alum will have the effect of LOWERING PH so careful monitoring is necessary when applying alum. Interestingly, if your water PH , general and carbonate hardness are low, simply bringing them up to recommended levels may result in much clearer water. If so, you may best forgo the alum treatment. "If it works, don't fix it" is sometimes the best rule.
  3. Alum use results in a sludge of precipitated particles that should either be vacuumed out or removed via a bottom drain.
  4. Alum is toxic to mollusks (snails, freshwater clams, etc.). At very low dose levels your snails, etc. may suffer minimal damage, but be forewarned that higher concentrations will result in their death. (Note: the same is true of potassium permanganate)
  5. Only apply alum when needed. Do not use it as a prophylactic. Use only enough to do the job.

How safe is alum? The jury may be out for a long time. If you eat a Big Mac at McDonald's, you have just eaten alum. (It's in the pickle Most public water supply systems add alum to reduce turbidity, so when you drink city water, you are already consuming alum ( ). (Some scientists are urging the use of ferric sulfate or calcium sulfate in municipal water systems instead of alum, because of the as yet unanswered link between aluminum and Alzheimer's disease ( ), ( Most baking powders contain alum as a leavening agent. (Clabber Girl, for example - http://www.

Ingestion of 30 grams (1 ounce) of alum has resulted in at least two human deaths ( Clearly, alum is a chemical that must be used wisely. But this is true of many things we use on a daily basis. Humans regularly consume hydrogen cyanide (present in many fruits), but in amounts too small to be of consequence. Household ammonia and chlorine bleach, both found in 90 percent of homes across America produce deadly chlorine gas when combined and both are poisons that must be kept away from children, but we regularly use them without untoward consequence. We need to keep that in mind whenever we object too strenuously to the use of chemicals. Objectivity is the key.

How much alum should you use? The answer depends on the specific situation. The amount of suspended particles in your water is the main variable. Here is a table of the recommended treatment levels for clearing muddy ponds that I found through research of the internet:

Alum Dosage for Clearing Muddy Fish Ponds
Unknown (1)147ppm
University of Fla. Ext.(2)2 - 5.5ppm
Texas agricultural Extension service (3)11 - 44ppm
Sweetwater Technology (4)(Sample case)60ppm

Now that we see a range from 2 parts per million to 147 parts per million are being used to clear fish ponds and lakes, let's compare that to the amounts added to potable water supplies for human consumption. According to WTA's Wold Wide Water Website, ( amounts added by municipal water systems range from 6 parts per million to 120 parts per million.

From this it would appear that any levels between 2 and 100 parts per million should be ok to drink, and by inference, ok for our fishponds. However, remember that the jury is still out regarding the safe consumption of any alum by humans.

How do we convert this information to useful dosing procedures for the use of alum in a watergarden? It works out like this:

Resulting Dose Level For Amounts of Alum Added to 1,000 Gallons of Water
1 Teaspoon1.2
1 Tablespoon3.5
1/8 Cup7
1/4 cup14
1/2 cup28.1
1 cup56.2

Don Steinback and Billy Higginbotham offer a method for determination of application rates based on test samples of water clearing rates: ( Their system involves making a slurry out of 1 level tablespoon of alum to 1 gallon of clear water. You stir until the alum is dissolved in a slurry. You then collect 1 gallon samples of the pond water in gallon glass jugs and add 1 tablespoonful of the slurry alum mix to one jug, two to another, and so on. Whichever jug clears within 12 hours tells you the amount of alum to use in your pond through the use of the chart. Their chart applies to farm ponds and is presented in pounds of alum per foot-acre of pond. I have converted their chart to home watergarden use as follows:

# TBLSP Slurry Added

Tablespoons Alum to apply per 1,000 gallons of Water

Resulting PPM













Notwithstanding the dosage levels proposed by the chart immediately above, my recommendation is, should you decide to use alum as a clarifying agent in your watergarden, that you begin with the 1 teaspoon per 1,000 gallon level and increase the dose as needed up to a maximum of 1 cup per 1,000 gallons.

Often, you will see alum also recommended as a plant soak to avoid bringing in snails, diseases, or leeches to your pond. The amounts recommended for that use run from 1 tablespoon per gallon opf water to 10 teaspoons per gallon of water. These ratios produce concentrations of from 3.5 parts per thousand to 11.7 parts per thousand. ("), (

URLs for the table references:

(1) Unknown. This information came from a website in the UK, and I have been unable to relocate it.





Last modified on February 28, 2000

<BGSOUND SRC="_images/bl-bayou.mid">