Wednesday, November 6, 2013

Water hardness and Fish Health

Water Hardness and Fish Health

Water hardness can have an important influence on many aspects of fish health. It has the power to influence the pH and pH stability of your aquarium water. One major effect that we have to talk about is the relationship between water hardness and fish osmoregulation.


In order to better understand what osmoregulation is, we need to tackle osmosis first.

Osmosis is a diffusion of water across a membrane from the hypotonic solution to the hypertonic solution. A solution with lower salt concentration is termed hypotonic and the solution with higher salt concentration is called hypertonic. Isotonic solution means that salt concentration in both solutions are equal. Have a look at how osmosis works in this video.

When salt concentration in the body of the fish differs from that of the external water, a concentration gradient is formed. Under this existing condition, there will be constant exchange of salt and water via osmosis to achieve equilibrium in concentration. The process of osmosis takes place on the thin membrane found on the gills of fish. Below is a figure taken from Campbell, A. Biology 1997 pg 149 regarding water balance of living cells.

When animal cell is immersed in isotonic solution, the flow of water in and out across the membrane occurs at the same rate. In this condition, the body cells are in a stable state and there is optimum function.

In hypertonic solution, the water from the cell moves to its environment leaving the cell shrivelled and may result to cell death.

In hypotonic solution, water will flow into the cell filling it like a balloon until it will burst. In plant cells, the rigid cell wall prevents bursting.


Osmoregulation is an important process to neutralize the natural forces of osmosis to maintain isotonic bodily fluids.

Being ectotherms, fishes are influenced by the conditions that exist in the surrounding water. As a result of osmosis, there is continuous flow of water into and out of the body of the fish. With this continuous water movement, the fish needs to maintain a constant concentration of its internal body fluid. This process which is involved in maintaining the concentration is called osmoregulation. Take note that the greater the difference in the concentration between the surrounding water and the body fluids of the fish, the greater will be the osmotic effect.

Since hard water is more concentrated compared to soft water, there will be lesser difference in concentration and consequently less water influx and lesser efforts undertaken for osmoregulation. When fishes work hard at osmoregulation as a result of unsuitable water hardness in fish aquarium tanks, it can eventually compromise fish health.

Watch the video below prepared by Paul Anderson, a science teacher and technology specialist at Bozeman High School. Paul explained the osmoregulation process of both fresh and salt water fish in an easy to understand manner.

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