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#136499 - 10/20/08 10:18 AM Hard vs Soft water
Ben Venturini Offline

Registered: 08/21/08
Posts: 15
Loc: Athens IL
What does hardness mean to my pond? My pond measured 222 ppm. Which means I have hard water. All this tells me is that the concentration of carbonates and bicorbonates originated from bedrock consisting of limestone and/or dolomite. But what does this mean for the overall health and water quality of my pond? Should I do anything now?

#136503 - 10/20/08 10:58 AM Re: Hard vs Soft water [Re: Ben Venturini]
ewest Offline
Hall of Fame 2014


Registered: 03/08/05
Posts: 18393
Loc: Miss.
From http://srac.tamu.edu/index.cfm?catid=25

Its about the Ca and Mg salts which may not necessarly be there if the alkalinity is from sodium bicarbonate.

Interactions of pH, Carbon Dioxide,

Alkalinity and Hardness in Fish Ponds


Water hardness is important to

fish culture and is a commonly reported

aspect of water quality. It

is a measure of the quantity of divalent

ions (for this discussion,

salts with two positive charges)

such as calcium, magnesium

and/or iron in water. Hardness

can be a mixture of divalent salts;

however, calcium and magnesium

are the most common sources of

water hardness.

Hardness is traditionally measured

by chemical titration. The

hardness of a water sample is reported

in milligrams per liter as

calcium carbonate (mg/L CaCO3).

Calcium carbonate hardness is a

general term that indicates the

total quantity of divalent salts present

and does not specifically identify

whether calcium, magnesium

and/or some other divalent salt is

causing water hardness.
Hardness is commonly confused

with alkalinity (the total concentration

of base). The confusion relates

to the term used to report

both measures, mg/L CaCO3. If

limestone is responsible for both

hardness and alkalinity, the concentrations

will be similar if not

identical. However, where sodium

bicarbonate (NaHCO3) is responsible

for alkalinity it is possible

to have low hardness and high

alkalinity. Acidic, ground or well

water can have low or high hardness

and has little or no alkalinity.

Calcium and magnesium are essential

in the biological processes

of fish (bone and scale formation,

blood clotting and other metabolic

reactions). Fish can absorb calcium

and magnesium directly

from the water or from food.

However, calcium is the most important

environmental, divalent

salt in fish culture water.
The presence

of free (ionic), calcium in culture

water helps reduce the loss of

other salts (e.g., sodium and potassium)

from fish body fluids (i.e.,

blood). Sodium and potassium

are the most important salts in fish

blood and are critical for normal

heart, nerve and muscle function.

Research has shown that environmental

calcium is also required to

re-absorb these lost salts. In low

calcium water, fish can lose (leak)

substantial quantities of sodium

and potassium into the water.

Body energy is used to re-absorb

the lost salts. For some species

(e.g., red drum and striped bass),

relatively high concentrations of

calcium hardness are required for


A recommended range for free calcium

in culture waters is 25 to 100

mg/L (63 to 250 mg/L CaCO3

hardness). Channel catfish can tolerate

low calcium concentrations

A low CaCO3 hardness value is a

reliable indication that the calcium

concentration is low. However,

high hardness does not necessarily

reflect a high calcium concentration.

But, since limestone is common

in the soil and bedrock of the

southern United States, it would

be reasonably safe to assume that

high hardness measurements reflect

high calcium levels.

A CaCO3 hardness value of 100

mg/L represents a free calcium

concentration of 40 mg/L (divide

CaCO3 value by 2.5) if hardness is

caused by the presence of calcium

only. Similarly, a CaCO3 value of

100 mg/L represents a free magnesium

value of 24 mg/L (divide

CaCO3 value by 4.12) if hardness

is caused by magnesium only.

These factors (2.5 and 4.12) are related

to the molecular weight of

CaCO3 and the difference in

weights between calcium and magnesium

atoms. Where hardness is

caused by limestone, the CaCO3

value usually reflects a mixture of

free calcium and magnesium with

calcium being the predominant divalent


Ideally, an aquaculture pond

should have a pH between 6.5 and

9 as well as moderate to high total

alkalinity (75 to 200, but not less

than 20 mg/L) and a calcium hardness

of 100 to 250 mg/L CaCO3.

Many of the principles of chemistry

are abstract (e.g., carbonate-bicarbonate

buffering) and difficult

to grasp. However, a fundamental

understanding of the concepts and

chemistry underlying the interactions

of pH, CO2, alkalinity and

hardness is necessary for effective

and profitable pond management.

There is no way to avoid it; water

quality is water chemistry.

#136674 - 10/21/08 09:43 PM Re: Hard vs Soft water [Re: ewest]
M Spinhirne Offline

Registered: 02/16/06
Posts: 210
Loc: Montgomery, Texas
What Erik said.
Keep in mind that water analysis are often (usually) reported as CaCO3 equivalents. That is, calcium may be reported as CaCO3 even if the calcium is not in the form of that salt. Calcium can be associated with sulfate for example, but be reported as the CaCO3 equivalent. To fully understand the water, you need to have a complete water analysis done. This will have all anions and cations reported and will give likely associated compounds. You need to know if the sample is reported as ion or as CaCO3 equivalent to balance the equations if this is not done.

#136721 - 10/22/08 09:32 AM Re: Hard vs Soft water [Re: Ben Venturini]
bobad Offline

Registered: 06/02/05
Posts: 2365
Loc: Eunice, Louisiana
 Originally Posted By: Ben Venturini
What does hardness mean to my pond? My pond measured 222 ppm. Which means I have hard water.

Since 222 is in the high normal range, it means your pond water is (barring other problems)healthy for the fish, and probably nice and clear.

#136780 - 10/22/08 07:10 PM Re: Hard vs Soft water [Re: bobad]
Greg Grimes Offline
Field Correspondent

Registered: 05/03/02
Posts: 3973
Loc: Ball Ground, GA
Bobad agree it probably means healthy. Not necccesarily agree with the clear part that depends on nutrients i.e. Phosphorus.
Greg Grimes


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