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We tend to take water very casually. We get drinking water out of the tap and don't think much about it. But to the fish, it is their whole environment. They can't live without good water. Water seems so simple, but it is an amazingly complex substance! And we can't tell if water is right without testing. Problems are usually colorless and odorless (you might be able to taste a problem in your pond water, but do you really want to do that?). Bad water can kill all of your fish in a matter of a few hours or less, and you probably wouldn't know the water was going bad unless you test. So to be safe, we have to test. Which test kit is most important? If you could only buy one test kit, which one would it be?

It is really a trick question, because it depends --

on whether you have city water or well water. What is the difference between the two? Chlorine (or chloramines) is in city water!

The first and most deadly chemical to be aware of is chlorine. It will kill fish faster than anything else. I have read that 1/1000 of the level in tap water is deadly to Koi. I believe it has killed more Koi Club member’s fish than any other water problem, or even any other problem of any type.
  • How often should you test? Always test after adding any amount of water. Read lengthwise along the test tube to insure you are seeing any amount of color in the test.
  • What is a safe level? Zero. Absolutely zero.
This is the second most deadly chemical we test for. One part per million is harmful to fish.
  • Fish produce ammonia just by being alive. Much of it comes from their gills.
  • The purpose of the biological filter is to convert ammonia to less toxic compounds.
  • A major enemy of new fish keepers is the new pond syndrome. A new filter is not able to handle the ammonia load from the fish. Ammonia levels can become lethal within just a few days.
  • Cure for high levels is water change - as much as 50% per day (don't forget to neutralize chlorine when adding water and test when you are done).
  • Problems with Nesslers reagent ammonia tests (because of chloramine and chemicals used to counteract it). Common chemicals we buy to neutralize ammonia will give false positive readings with Nesslers reagent ammonia tests.
  • How often to test? Daily or more often in hospital tanks. Daily in new ponds or with new filters (until ammonia spike drops to safe levels). Anytime things change with filters or fish don't act normal.
  • What is safe level? Zero.
This is the next step in the nitrate cycle. It's less toxic than ammonia, but not by much. As soon as ammonia levels start to go down in a new pond, the nitrite levels will shoot up.
  • Cure for high levels is water change (test for chlorine)
  • Salt supposed to relieve its symptoms.
  • How often to test? Daily in new ponds or with new filters (until nitrite spike drops to safe levels). Anytime things change with filters or fish don't act normal.
  • What is safe level? Zero.
This is the final stage of the nitrate cycle. Included here for completeness. Is generally considered harmless in ponds, although have had one report of where fish didn't eat, test readings were off the scale, and when corrected, the fish ate normally.
  • Cure for high levels is water change (test for chlorine).
  • What is safe level? At least 100 parts per million (The Tetra Encyclopedia of KOI says 500 ppm is not harmful--I wouldn't let it get that high!).
The name of this test is misleading in my opinion. Some test kits calls it Carbonate Hardness, but that is not to be confused with hardness, which is the level of the positive ions Calcium and Magnesium (Tetra calls that General Hardness). For all practical purposes, total alkalinity is the level of the bicarbonate ion in the water one half of the old Arm and Hammer baking soda (sodium bicarbonate). It is present in adequate levels in almost all water sources.

The problem is, it is used up in the biological filtration process, which produces acid! It is absolutely vital for the health and safety of fish. Since it is used up, it must be replaced. Because it is present in almost all water sources, water changes, if done often enough, can keep levels correct. Other sources are to add Arm & Hammer baking soda whenever needed, or provide natural sources such as a concrete pond, whole or crushed oyster shells or crushed coral in the filter.

The bicarbonate ion (HCO3+) is very important as a buffer in the water. It keeps the pH around 7.4 to 7.8 or even higher. At a low pH, the biological filter starts to shut down, the ammonia shoots up, and the fish die.

Safe levels are anything over 40 parts per million (some experiments indicate that the filter starts to slow down below 90 ppm. This will keep the pH in that 7.4 to 7.8 range. Lower pH is too low. Higher pH is of no concern, our pond tends to run 8.0 to 8.2 in the winter when the biological filter is not doing very much. How to lower pH if it is too high? This is another trick question. NEVER do anything to lower the pH! All you will do is destroy the buffers (bicarbonate) in the water. Biological action will then cause pH to plummet.

To increase total alkalinity, 1 pound of sodium bicarbonate will raise 3,333 gallons 20 ppm.

This is probably on the top of most people's list of test kits to buy. Not even hardly on mine. In my opinion, pH is only a secondary indicator of what's going on. The real test of what's happening is the level of Total Alkalinity, and that's what I test for.

What is the proper level? 7.4 to 7.8 or even 8.4.
A pH of 7.0 is too low.

Most water supplies have sufficient Calcium--a minimum of 100 ppm. This is tested with a hardness or General Hardness test kit. This is needed for the bicarbonate to complete its job as a buffer and is also needed by the fish and the filter bacteria. A few ponds in North Carolina and southern Virginia have too little calcium, and these ponds have extremely high pH that is not reduced by adding sodium bicarbonate. I believe that this is because the water becomes saturated with the carbonate ion, which wants to marry up with calcium and become a rock. If there is no calcium, it just hangs around, but in doing so it prevents more bicarbonate from releasing a hydrogen ion and thus lowering the pH. There have also been reports of such water in New Jersey. The cure is to add calcium chloride, which is usually available in swimming pool supply stores.
Most experienced Koi keepers say that you can't have too much oxygen in a Koi pond. They have waterfalls, air stones in filters and ponds, and venturi jets to provide aeration. The air we breathe has almost 21% oxygen, but it just barely dissolves into water. Depending mostly on temperature, the saturation level of oxygen in water is only about 7 to 13 parts per million (compared with about 210,000 ppm in air). So fish are constantly challenged to find enough oxygen. The problem is worst when the water is the warmest in the summer, because that's when the fish's metabolism and oxygen needs are the highest, yet the water holds the least oxygen.

What is the proper level? The Tetra Encyclopedia of Koi says that Koi require at least 6 ppm of oxygen. I have read that oxygen should be kept within 2 parts per million of the saturation level for the particular temperature. How often to test? Whenever anything changes or the fish act sluggish or congregate at water inlets or base of a waterfall.

Copper is extremely toxic to Koi. But you shouldn't have to test for copper because you shouldn't have anything copper in the pond. Is that right? No, because most people have copper pipes in the house water system. With city water, you might want to flush water out of a pipe before adding it to your pond, especially if it has been standing in the pipe for a long time. If you have well water, it is absolutely necessary to flush that pipe every time you add water to the pond. Well water tends to be acid as it comes out of the ground because of an abundance of carbon dioxide dissolved in the water. The acid brings copper into solution and can cause real trouble.

If you decide to heat your pond with a pond heater, be careful of the heat exchanger. Several members have gotten in trouble with heaters that have a copper heat exchanger. They should be OK, but only if you do everything right. Especially important in this case is maintaining the proper level of total alkalinity (because the bicarbonate ion will precipitate copper).

Iron is toxic. Ann Tellford says 0.05 ppm (that’s 50 parts per billion) is the safe level, but only if other harmful metals are not present. A swimming pool store may be able to test your water for iron.

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