Defining water quality

...do you meet the five point standard?

The single, most important factor affecting fish health and influencing disease in fish ponds and tanks is water quality. Raised levels of ammonia or nitrite, sub-optimum pH and water hardness levels or a high level of organic pollution will be stressful to koi and other fish; predisposing them to disease. If we are to create healthy, optimum conditions and prevent disease, it is important to be clear what is actually meant by good water quality.

Water quality - not clarity

Many koi keepers and other fish keepers tend to judge water by its clarity.  Whilst clear water is obviously desirable, just because water is clear it does not necessary mean that it is good water. After all, concentrated hydrochloric acid is crystal clear and so is water heavily laced with arsenic! Fish want a little more than just clear water – indeed, they possibly prefer it slightly murky and green. If we are serious about providing good conditions the question we should ask is what do fish want?

1. Low ammonia and nitrite

Fish are constantly polluting their own environment and producing ammonia. Both ammonia and nitrite are highly dangerous, causing stress and physical damage to sensitive tissues. A major, major requirement of any fish keeping system is no detectable levels of either. This particularly applies to new set-ups (new pond syndrome) and heavily stocked koi ponds. Biological filtration may be needed to maintain optimum levels.

2. Chemically clean water

The water should be chemically clean and free of chemicals such as pesticides, chlorine, heavy metals, organophosphates and chemicals used to treat fish diseases. The presence of any toxic chemicals, even at fairly low levels, may be harmful. OK, we do have to treat fish from time to time - the point is to realise that any chemical treatment will compromise water quality, and for the duration, conditions (from the fish’s perspective) will be less than optimal.

3. Water hardness, pH and temperature

Different species of fish have specific requirements for essential water parameters such as pH, water hardness, alkalinity and temperature. Conditions outside of what are fairly narrow limits are liable to create stress.  Water that fails to meet these criteria cannot for obvious reasons be considered good water quality

4. Low levels of organic pollution

In addition to fish waste, the pond or tank is also being continuously polluted with uneaten food, algae and other detritus. As this organic matter decomposes it produces many organic and inorganic compounds. Biological filtration will take care of ammonia and nitrite, but there may be a build up of dissolved and particulate organic compounds. High levels of organics (POCs and DOCs) can create conditions that encourage disease, parasites and opportunistic bacteria. Water with high levels of organic matter cannot be considered good water quality.

5. Stability not fluctuation

Dr. Erik Johnson veterinarian in Marietta Georgia for dogs, cats, fish AND ponds.

Depending on the water chemistry, stocking levels and pond design, it is possible to have substantial fluctuations of pH, temperature and other parameters over a 24-hour period. Constant changes - even if they stay within the preferred range are liable to be extremely stressful, as the fish have to constantly adapt to changing conditions. An example might be pH that varies between, say 7 in the morning, rising to 9-10 in the evening on a hot sunny day. Apart from stressing the fish, it will have other implications for other water chemistry aspects such as ammonia and many common disease treatments. Water that constantly fluctuates in quality and conditions cannot be said to be good water qua


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Hopefully, this overview has given food-for-thought about what we mean by good water quality. Based on these criteria, how many of us can honestly claim to have excellent water quality all of the time? The rest of the pages in the water quality section explain more about water; how it is formed, how variations in quality or chemistry can affect fish health and what steps to take to create optimum conditions for your fish.

Mardel Clout is safe in tanks and ponds, minimal to no effect on plants and bacteria, safe for scaleless and sensitive fish. Dosing in this website.

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