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Water Chemistry for aquarists

Discussion about fish health and water chemistry.

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Post Thu Feb 01, 2018 11:24 am
Passionfish Level 20 Member
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Posts: 12302
Location: apple valley, mn
In discussing water chemistry, hard water refers to divalent ions and calcium is responsible for most of the hardness in water. Germans were the first to define hard water with degrees, 1 degree is 10 mg of calcium oxide (CaO) per liter water. One degree general hardness is abbreviated to dGH or if converted ppm is 17.8 ppm. Measurement of calcium, magnesium and other divalent ions is a measurement of permanent hardness.

Hardness in USA is defined in grains per gallon. One grain is 65 mg of calcium carbonate per gallon of water. A grain is also convertible to ppm and is 17 ppm calcium carbonate.

Carbonate hardness is a valid term and is also called temporary hardness. However, carbonate hardness leads to confusion in practical discussions of water hardness or softness. IMO, carbonate hardness should be replaced with carbonate alkalinity. Aquarists have no interest in true carbonate hardness. Further, when pH is below 8.5 then carbonate hardness and carbonate alkalinity are the same value. It is carbonate alkalinity that determines whether water is alkaline or acidic and this is what concerns aquarists. Carbonate alkalinity is expressed in ppm. Measuring calcium carbonate in solution, one degree KH is equal to 17.85 ppm carbonate.
Note other ions contribute to alkalinity but the concentrations of these other ions in tap water is low or not present; therefore instead of referring to total alkalinity, carbonate alkalinity is considered responsible for all alkalinity. Alkalinity measurement is how much acid is needed to neutralize all alkaline ions occurring at pH 4.2

I suspect the use of permanent hardness for calcium and temporary hardness for carbonate leads to much confusion. Therefore aquarists should avoid thinking of temporary or carbonate hardness and instead refer to carbonate alkalinity.
Review: GH only measures calcium (hardness), KH only measures carbonate (alkalinity) and grains measures both calcium and carbonate (when not specific, grains are referring to calcium measurement).

Why is this important to an aquarist?
1. Hard water is mostly calcium and can be determined (measured) with kits and probes. Various fish and plant species either prefer or do not enjoy hard water.
2. Alkalinity is mostly carbonate and is also determined by kits and indirectly with a pH meter. Large concentrations of carbonate will push pH above 8.0 and keep it there unless acid is added. Added acid pushes pH down. At pH 4.2, all carbonate has been converted to non-alkaline ions. Adding same quantity of acid will now result in a large decrease in pH. Various species of fish and plants prefer or do not enjoy acidic water.

It is by knowing if our tap water, tank water concentration of calcium and carbonate determine what we can and cannot keep successfully.
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Post Thu Feb 01, 2018 12:00 pm
Passionfish Level 20 Member
Level 20 Member

Posts: 12302
Location: apple valley, mn
Once water chemistry for an aquarist is understood, the next question becomes what to do about it.

Clearly to achieve acidic water carbonate must be removed or neutralized with acid.
Also clear to achieve soft water, calcium must be removed. There is no method aquarists can use to neutralize calcium. It must be removed to create soft water.

Start with calcium, it can be removed by reverse osmosis or by deionization. A home water softener is a deionization column that only binds cations such as calcium, magnesium, sodium, etc. Carbonate ions are not removed by a home water softener. The resin of home water softener binds calcium from tap water and sodium is released from resin to replace calcium in tap water. It is advised not to drink or cook with water from home water softener due to increased sodium; however, I am not aware of any medical studies showing definitely harm from ingesting soft water.
Reverse osmosis (RO) will also remove calcium from tap water and many more ions also, including carbonate. Very few ions pass an RO membrane. Ions that do pass through RO membrane can be subjected to a mixed bed resin to create pure water with no ions. Aquarists like pure water when they wish to create water they want for fish, plants, corals, shrimp etc.
Before proceeding, a little more about resin columns and RO membranes. Resin column of home water softener eventually is saturated with calcium. There are formulas to predict when this will occur and testing will confirm when the column is saturated with calcium. At that time point, the water softener regenerates resin by flushing calcium off resin with very high concentrations of sodium chloride in the wash water. There is a concern of this sodium chloride from a flush of resin entering wastewater system but to date, there are no regulations concerning the use of home water softeners and discharge of large amounts of sodium chloride into the wastewater system.
Why would aquarist be interested in a home water softener? When calcium is removed by RO membrane, the life of membrane is reduced. Therefore, using softened water to feed to RO unit will prolong the life of RO membrane. This is primarily a selling point for home RO units providing drinking water but an aquarist could benefit from using softened water to RO unit as well. RO membranes are continually flushed with feed water to remove ions that membrane is rejecting. If continuous flushing of RO membrane fails, RO system will stop creating RO water and it is likely RO membrane will need to be replaced. Testing of RO water may be helpful.

Presume we have RO water at pH of 7 or close to it. Water at pH of 7 is not hard or soft, it is not acidic nor alkaline, it is neutral.
A reef keeper will add salts to create an ionic environment that favors coral. A plant keeper may add nothing to water may infuse with carbon dioxide. Addition of CO2 will acidify water and some alkalinity may need to be added to keep pH from becoming too acidic. Then there are fish keepers that want both soft and acidic water, usually, they are attempting to replicate blackwater. Blackwater is created from wood and leaf litter falling into water that is not exposed to calcium carbonate. The origin for most of this water is rainfall, which may be slightly acidic and is calcium free. Leaf litter and wood release tannins that create tea stained water or black water. Tannins are slightly acidic and can bind small amounts of calcium if present. IMO, sphagnum is a great source of tannins for the aquarist. Wood total immersed may also release tannins as well but not to the degree of sphagnum.
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Post Thu Feb 01, 2018 12:40 pm
Passionfish Level 20 Member
Level 20 Member

Posts: 12302
Location: apple valley, mn
What I have in mind is this:
Use a home water softener to remove calcium from water. Calcium is gone, water is now soft.
Then use infused carbon dioxide to break remaining carbonate, now water will be acidic.

Issues:
Home water softener is more expensive than RO unit. Soft water may have other uses in the home such as soaps are more effective in soft water than hard water, calcium salts form scale in hot water heaters.

Infusing CO2 in a tank may result in death of fish if dissolved CO2 levels are not monitored closely.
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Post Fri Feb 02, 2018 2:07 pm
d33pVI Level 1 Member
Level 1 Member

Posts: 16
Passionfish wrote:
What I have in mind is this:
Use a home water softener to remove calcium from water. Calcium is gone, water is now soft.
Then use infused carbon dioxide to break remaining carbonate, now water will be acidic.

Issues:
Home water softener is more expensive than RO unit. Soft water may have other uses in the home such as soaps are more effective in soft water than hard water, calcium salts form scale in hot water heaters.

Infusing CO2 in a tank may result in death of fish if dissolved CO2 levels are not monitored closely.


Dissolved CO2 reacts with water to form carbonic acid, which only provides a temporary drop in pH. Carbonates present will affect how much CO2 can dissolve, but alkalinity does not change. Stop the flow of gas and pH and kH will be the same as where you started.

Post Sat Feb 03, 2018 4:16 am
Passionfish Level 20 Member
Level 20 Member

Posts: 12302
Location: apple valley, mn
d33pVI wrote:
Dissolved CO2 reacts with water to form carbonic acid, which only provides a temporary drop in pH. Carbonates present will affect how much CO2 can dissolve, but alkalinity does not change. Stop the flow of gas and pH and kH will be the same as where you started.


Yes, you are absolutely correct.
Not all CO2 reacts with water to form carbonic acid, there is an equilibrium of dissolved CO2 and carbonic acid in the water.
It is the dissolved CO2 that could be dangerous for fish. As the concentration of CO2 at gill approaches or exceeds the concentration of CO2 bound to fish hemoglobin, the hemoglobin will not release CO2 and cannot bind O2. Fish dies.

It is difficult to measure the concentration of CO2 dissolved in water. It may be possible to use carbonate concentration and the desired pH to calculate CO2 concentration. I will have to check to see if that is possible.
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