Coming Up for Air

Of all the elements found on Earth, oxygen is probably the most important. It is vital to almost every living organism, including pond fish.

If you’re installing or maintaining ponds with live fish in them, you should understand some basic things about how oxygen works in an aquatic environment.

Our fishy friends need oxygen as much as we do; they just get it in a slightly different way—through their gills. Fish and aquatic plants both uptake dissolved oxygen from water. The oxygen in and around a pond is what keeps it bursting with life.

Of course, the chemical name for water is H 2 O, but the compound’s ‘O’ portion alone isn’t the only oxygen that’s present in a pond. And, just because oxygen is part of what makes up water, doesn’t mean that there’s automatically enough of it in a body of water to sustain aquatic life.

How oxygen gets into a pond

The oxygen in a backyard pond comes from several different sources; most commonly, from good old-fashioned atmospheric absorption. Splashing, as in a waterfall, and other agitation at the surface increases the absorption of oxygen into the water, because it expands the surface area. More surface area equals more oxygen.

Aquatic plants bring additional oxygen into a pond, but we can’t rely on them to do all the work. While plants with submerged foliage can contribute massive amounts of oxygen when the sun shines on them, it’s a double-edged sword, since they also use it.

Fast-growing plants with lush foliage that grows under the water line, including anacharis, elodea and cabomba, are some of the best oxygenators around. So is that not-sobeloved plant called algae.

Like all plants, aquatics use carbon dioxide, and produce oxygen.

But, because they grow during the night, drawing on energy they store during daylight hours, they reverse that process after dark, producing carbon dioxide and consuming oxygen. In other words, when nighttime hits, submerged plants (a.k.a. oxygenators, or submerged aquatics) are not fishs’ best friends.

Once the oxygen is present in the water, it’s used by the aquatic plants and animals in it for respiration. Respiration is the key to their growth and survival, just as it is to ours. Bacteria also use some of the oxygen, to help break down dead plant material.

Exactly how much oxygen is needed for fish to survive? A level of five parts per million (PPM) will allow fish to live for a few days; but eight PPM is much better, and 11 to 14 PPM is best. Keeping fish means maintaining a suitable oxygen level for them to thrive. It’s as vital as the very water in which they live—but water alone is not enough.

The role of water temperature

When it comes to oxygen levels, cooler is better. In case you don’t remember this from Chemistry 101, colder water—under 60° F—dissolves, or carries, more oxygen. Pond owners who live in colder climates, who seem to get the short end of the stick in other areas of water gardening, have the edge in this department.

If cooler temperatures allow more oxygen, then obviously, heat reduces it. That’s why you need to be careful with fish during cleanouts. Regardless of whether you’re in a warm or a cool climate, putting a client’s fish in a tank or tub in the heat of summer while you clean the pond is risky. If you don’t aerate or agitate the tank or vat you’re using, or you leave it in the sun, those fish will be in danger of having their oxygen levels drop too low.

This can be prevented by simply aerating or agitating water that’s over 75° F. An air stone makes this task simple and easy. While you’re at it, why not get some extra help from Mother Nature by simply picking a nice, shady spot for the fish to hang out while you do the dirty work?

Other ways oxygen levels can drop

Sizzling summer temperatures aren’t the only villains in the Great Oxygen Robbery saga. There are several other common ways that the O 2 levels get depleted; fish and plant respiration, for one.

That’s why it’s so important to make sure you don’t overstock a pond. Give them some breathing room. One inch of fish per square foot of pond is the recommended number. Also remember that fish grow, and you need to give them space for that, too.

Bacteria, even the kind you want, are also culprits in oxygen consumption. In fact, beneficial bacteria have especially voracious metabolisms when it comes to sucking up oxygen; they consume more than the fish could ever dream of. It’s kind of ironic that the very organisms you need and want in a pond are the things that use up the most oxygen.

There are other processes that can cause dissolved oxygen levels in a pond to drop. These include the presence of decaying plant matter, and treatment with chemicals. Decaying algae is especially bad, consuming massive amounts of the precious element as it rots away. That’s why algal blooms in lakes often lead to massive fish kills.

Sick fish really need you to watch their oxygen levels. The chemicals used to treat common fish diseases are known to consume a lot of oxygen. If you’re the one treating them, it’s a good idea to agitate the water while you’re doing it.

Finally, the depth of a pond plays a significant role in the level of available oxygen. Ponds over five feet deep, for example, will have low dissolved oxygen levels at the bottom.

This will be true unless there is a means to bring the bottom layer of water to the surface.

The oxygen in a residential pond usually stays at a decent level, as long as there’s sufficient water movement, such as is supplied by a robust waterfall. Remember, the main way that oxygen enters into, and mixes with water, is via contact with the atmosphere.

An overdose of O?

“Can there be such a thing as too much oxygen?” This question lurks in the back of many pond owner’s minds, and the answer is, “Yes, but hardly ever.” A singular, very rare condition can occur that creates an overabundance of the element, and only in the summertime.

If fish are kept in a deep pond in full sun, and the bottom of that pond is covered in algae, it is possible that, with very clear water, the algae could super-saturate the water column with micro-dissolved oxygen.

If there is no shade, and no way for fish to leave this water column, then the saturated, dissolved oxygen may cause the fish to develop something called ‘gas bubble disease.’ It damages their tissues, and can even kill them.

It’s rare, because it can only occur in the warmer months, when oversaturation with oxygen is virtually impossible. Fortunately, since warmer water generally contains less oxygen, and water is usually warmer in the summer, it would be extremely odd if this were to happen.

Test, test, test

If you ever observe pond fish near the water’s surface, seemingly gasping for air, that’s exactly what they’re doing! Don’t wait—check the oxygen level, and if it’s low, do something to fix it, immediately. If you’re maintaining ponds on a regular basis, get an oxygen-level test kit and bring it with you whenever you make service calls.

If the oxygen level in a pond is good, there’s no need to try to improve it. The fish and plants have become used to their surroundings, and have probably already adjusted to the pond’s chemistry. Remember that pond pets are just like you, living and breathing the same air... just a little bit differently!

EDITOR’S NOTE:: Ed Beaulieu is chief sustainability officer at Aquascape, Inc., www.aqua