Who’s on pHirst?

(This article is being written during the Super Bowl. (Actually Super Bowl XX, 1985. Bears vs. Patriots. Got it on tape 18 years ago.) So a sports metaphor is appropriate, even if it is baseball!)

Alright, keep your hands on the newsletter. Step away from the door and sit down. Do not run away. We are your friends, and a discussion of pH is not going to hurt you. Much.

Every time pH is mentioned in ponding circles, especially in print, the sound of eyeballs glazing over is well-nigh deafening. I don’t know why this is, but it seems to be one of the hardest things to explain in any fashion that leaves the listener (or reader) conscious and mentating. I’ve seen definitions involving ions and balances and atoms of hydrogen and oxygen. I’ve seen demonstrations of formulas worthy of Einstein and Planck at their most opaque, but I don’t know that I’ve seen much that was useful for the average backyard ponder.

I guess the best way to start is with some generalizations. These are not necessarily the total truth, but as a very wise man once said, they are very useful lies.

  • Every pond exists with a balance of acid and alkali (base) components as constituents of the soup that we call pond water.
  • Acids are things like lemon juice, vinegar, and the stuff inside your stomach.
  • Alkalis are things like quicklime, concrete,  and sodium hydroxide.
  • Both, in high concentrations, can eat holes in things. (Did you really need to know that? No, but it’s fun, so I said it anyway.)
  • Both are present to one degree or another in our pond water, and are either a part of the water itself or of the minerals (or pollutants!) dissolved in the water.
  • If you have equal amounts of acid and base in your water, these balance out and this state is called “neutrality”.
  • pH is a convenient way of measuring this balance, with a pH of 7 reflecting the neutral state. Numbers less than 7 indicate a more acid state, and those above 7 a more alkaline condition.
  • pH changes are logarithmic (don’t panic!) To quote Norm Meck, my Chemistry guru, each unit of change is 10 times more than the previous one. pH 8 has 10 times more base than pH 7, and pH 9 has 10 times more base than pH 8 (100 times more than pH 7!). The same is true for acid at pH values below 7. pH 6 has 10 times more acid than pH 7, and so on.
  • Dissolved materials, such as ammonia, are affected by the pH balance in our ponds, and their ability to harm our fish depends on the pH level.
  • The bacteria in our filters function best within a narrow range of pH, and sudden changes, especially in the acidic direction (low pH) can wipe out our bioconverters.
  • Koi are tolerant of a wide range of pH, as long as it is stable.


There, that didn’t hurt too much, did it?

It isn’t really necessary to completely understand every nuance of the chemistry and physics of pH, but it is important to be able to measure it accurately on a regular basis, to understand the importance of maintaining a stable pH, and the consequences of failing to do so.

Measuring pH

A trip to any pet supply store will present you with a number of choices of pH kits. These range from little bits of paper only one step removed from the litmus paper you played with in science class, to tablets in little indestructible foil pouches, to bottles of mysterious fluids. Regardless of which you buy, a few things need to be considered before you plunk down your money.

  1. Is the kit you are buying sensitive in the range you need? Ideally, ponds should operate in near-neutral water conditions (7.0-8.5). In reality water pH can vary anywhere between 6 and 8.5 and still be perfectly healthy. The pH can vary with time of day, water source, additives, pollutants, plantings and a host of other influences. As long as the changes are gradual, koi tend to be pretty tolerant. Bio-converting bacteria tend to lose efficiency at low pH, and pH above 9 is thought to damage koi kidneys. Many pH test kits vary in accuracy and are accurate in only a specific range of pH. A kit with a very wide range may not give accurate readings in the range you need. A very accurate kit within a narrow range of pH won’t let you know if you are in trouble outside that range. In general, any kit that will give you easy-to-detect color changes in a range between 5 and 10 (“wide-range”) is satisfactory for pond use. For those of us with color-blindness or a need for micro-management, electronic pH meters are available. These require frequent calibration and maintenance and are often fragile. The average Joe Ponder need not make this investment.
  2. Are your reagents fresh? Testing materials bought from pet supply stores are often ordered months in advance, and can sit in storage areas (often without temperature or moisture protection) for weeks to months. Before that, they can reside in warehouses for as long or longer. Even specialty suppliers have been known to send old and inaccurate reagents to hobbyists. Your best bet is to order from a supplier with a good reputation for reliable reagents and a mechanism for tracking lots of reagents by date of manufacture. Most pH kits have a shelf life of about two years. After that they should be replaced.
  3. Is the kit easy to use? Complicated testing procedures will discourage you from keeping up with your monitoring program. Most good pH reagent kits are one-step and can be read almost immediately. Any kit with instructions written in engrish as translated from the original Japanese by a native Tagalog-speaking Scotsman should be gently replaced on the rack, as should any kit that requires endless shaking of an insoluble and indestructible tablet.
  4. Will the kit last the season? Many reagents are sensitive to deterioration by moisture, heat and/or sunlight. Make sure your kit is appropriately packaged and the quantities are sufficient to last you long enough to make the purchase worth your while. If you are buying paper strips, for instance, water-tight packaging is a must!
  5. Why do I have to measure pH, anyway? Ah, now. On to the next section.

Maintaining stable pH

pH is not a fixed value in any pond. It is a constantly changing dynamic balance between acid and base that can be affected by a multitude of environmental influences. Without some stabilizing mechanism, pH can vary widely and rapidly, not a good thing for the fish or the filter. What keeps this from happening is the presence of dissolved salts of magnesium, calcium and other minerals which are capable of swapping acid and base components within the pond environment. These are present in most water sources, so that frequent water changes can replenish these salts, which are broken down and consumed by organic acids produced by common pond bacteria. If you think of these salts as a big sponge, sopping up the acids produced by rainfall, fish, bacteria and other sources, and minimizing the effect the added acids have on the pond environment, you are close enough to the truth to make it useful. We call this “pH cushion” a buffer, we measure it as alkalinity,and the action of alkalinity on a pond is it’s buffering capacity.

The term we use for the strength of this protection against pH shift is Alkalinity, measured in parts per million (ppm), and test kits, subject to the same questions and requirements as your pH kit, are widely available. Ideal alkalinity is around 100 ppm; a range of 50-200 is acceptable for most ponds. This means your test kit will need a range of 0-200 to be useful.

Attempts to change pond pH rapidly by the addition of acid or alkali directly to the water usually result in major stress to the fish and filter. As long as the pH is stable, ranges of pH between 6 and 8.5 are well-tolerated, and it is a better idea to find ways of stabilizing your pH. This is actually fairly easy to do by watching your alkalinity, and supplementing it if necessary. Supplements can be as simple as a few limestone flags, or if a more rapid correction is needed, a “biscuit” of plaster of Paris placed in the pond and replaced as it is consumed works well. Emergency corrections of alkalinity can be made with simple Arm & Hammer baking soda (pure sodium bicarbonate).

So… What if I Don’t?

Wide shifts in pH over short periods of time are stressful to both fish and filters. Water that is very alkaline (pH 9.0) has been suspected of damaging koi kidneys. Since the addition of acids to ponds is very tricky, a water source that is less alkaline may be preferable.

Sudden reductions in pond pH can occur in liner ponds without access to rock or other sources of salts, as alkalinity is consumed by the production of organic acids, reducing its buffering capacity. When this happens, the pH can drop suddenly and without warning, a phenomenon called a “pH crash”, which can drop the pond pH to 5 or less. Since the bio-converting bacteria in your filters lose efficiency at pH below 6, acid conditions of this intensity essentially turn off your filter’s ability to process the ammonia your fish are still making. It will now take weeks to get it back, since very low pH will kill a significant portion of your bacterial population. Your fish are protected in the short run because ammonia becomes ionized at low pH, and is considerably less toxic. Trying to correct the pH towards neutral without first binding the ammonia with an additive will de-ionize the ammonia and kill the fish.

Overall, weekly monitoring of pH and alkalinity, and supplementing the alkalinity as needed, can save you a lot of trouble and heartbreak, and get you that much closer to an ideal pond.

Respectfully submitted,
Bob Passovoy