The Skeptical Fishkeeper: Aeration and Oxygenation
Author: Laura Muha
A few weeks ago, I noticed that the airstone in my 20-gallon tank was starting to fizzle. Instead of perking away merrily, as it had when I’d added it to the tank some months before, it was emitting a feeble stream of bubbles, almost like a carbonated soda that’s been left too long in the sun. No big surprise there; airstones—whether wood, ceramic, or actual stone—are full of pores that over time tend to get clogged with mineral deposits and algae. And while I’ve heard of people recycling them through a multi-step process that includes boiling them in vinegar and water, scrubbing them, and forcing air back through them, my feeling about that is: why bother? When compared to the many other things that need periodic replacement in aquaria, a 50-cent airstone is hardly a big deal. However, you know me (or at least you do if you read this column regularly), and I like to ask questions; and if I can get a column out of it, all the better. Therefore, I present to you the topic of the month: air stones and other forms of added aquarium aeration.
• Do Fish Need Oxygen?
• Oxygen in Water, Explained
• How Tank Dimensions Affect Oxygen Levels
• What Happens When Oxygen Levels Drop?
• An Anecdote About Overstocking
• How Much Oxygen Do Fish Need?
• The Basics of Aeration Devices
• Are Airstones Necessary?
To find out if airstones and added aeration are really necessary, let’s start with the basics. Fish need oxygen to fuel their metabolic processes, and like us, they will die if deprived of it for long. However, the similarities end there, because, at the risk of stating the obvious, fish live in a very different environment than we do. Even under the best circumstances, there’s 95 percent less oxygen in water than there is in air. Plus, water is 800 times more dense than air and 50 times more viscous. So, fish have to expend a lot more energy “breathing” water—that is, pumping it through their gills—than we do breathing air. The structure of a fish’s gills, however, helps to make up for this because the feather-like filaments and lamellae create a vast surface area that enables the fish to extract about 80 percent of the oxygen from the water that passes over them. By comparison, our lungs extract a paltry 25 percent of the oxygen contained in every breath we take—and that’s only if we’re in peak condition. However, we do have one thing going for us that fish don’t have, in that the level of oxygen in our atmosphere remains a relatively constant 21 percent. So barring an oxygen-consuming disaster such as a fire, we can count on plenty of it being available to us every time we inhale. Fish can’t count on the same, because they live in an environment in which the amount of available oxygen can vary, sometimes considerably, from one moment to the next depending on things like barometric pressure, salinity, the presence (or lack thereof) of plants, the weather, the depth of the water, the time of day, and especially temperature. But I’ve always found that concept to be a little confusing; I mean, aren’t water molecules themselves 1/3 oxygen?
In search of an explanation, I called my favorite chemist, who also happens to be my father, Dr. George Muha, a professor emeritus of chemistry at Rutgers University. He explained that while water itself is composed of two hydrogen atoms and one oxygen atom, the oxygen in the water molecule itself is not the oxygen the fish are breathing. That’s because—here I could almost hear him thinking “Duh!”—the oxygen in water molecules is already tied up making, well, water. Instead, the oxygen that fish are breathing is essentially the same atmospheric oxygen we humans breathe; it’s just that when it comes into contact with water, it dissolves into it in much the same way that, say, sugar does. Sometimes, this contact point occurs beneath the surface. Aquatic plants
, for instance, release oxygen during photosynthesis, although this happens only when the sun is out. (At night and on cloudy days, they pull oxygen from the water instead, thus explaining why oxygen levels can drop overnight or in bad weather.) And if you have an airstone, an air-driven box filter, or some kind of decorative bubble device in your tank, the bubbles it produces are pumping some oxygen into the water as well.
By far the largest amount of oxygen present in most tank water has worked its way down from the surface, which is one of the reasons that a wide, shallow tank is a better choice for most fish than a tall, narrow one. Compare, for instance, a 20-gallon long tank with a 20-gallon regular. Both hold exactly the same amount of water, but their dimensions are different: the regular is 24 inches in length, while the long is 30 inches in length; both are 12 inches wide. If you multiply the length by the width to get the surface area of each, you’ll see that the 20 long has a much bigger portal through which oxygen can enter the water than the 20 regular: 360 square inches vs. 288. And the long tank has another thing going for it as well. It’s shallower than a 20 regular (12 inches deep versus 16). So oxygen entering the tank at the surface will work its way to the bottom of a 20 long faster than it will a regular 20, if all other things are equal. However, all things are not always equal when comparing oxygen levels of various tanks, because of all the other variables that come into play. Salt water, for instance, doesn’t hold as much oxygen as fresh water, and the same is true of warm water when compared to cool. That’s because water molecules become increasingly agitated as the temperature rises, and the more they bounce around, the harder it is for gases to dissolve in them and the easier it is for the oxygen that’s in there to get bounced out.
Other substances that aquarists sometimes add to water (medications, for instance) also can cause oxygen levels to drop. That’s because some such substances bond so tightly to water molecules that—to grossly oversimplify an incredibly complicated dynamic—it effectively decreases the amount of water in solution that’s available to hold the oxygen. When this happens, fish have a number of ways to compensate. They can push water through their gills faster (the fish equivalent of panting); they also tend to hover near the surface where the oxygen content is higher. Over the long-term, they can increase the number of red blood cells and the concentration of hemoglobin within them to more efficiently transport oxygen to their tissues. And in truly desperate situations, they may do what’s known as piping—swimming head up at the surface, opening and closing their mouths as they suck at the oxygen-rich skin of the water, something I witnessed the first summer I kept goldfish.
Back when I wasn’t experienced enough to understand stocking limits, I put too many goldfish in a too-small pond. When I went outside to feed them one morning I found them all hanging at the surface opening and closing their mouths as if they were gasping. A frantic call to the pond shop identified the likely problem. It was very hot, so the water wasn’t holding much oxygen to begin with; it also was overcast, so the plants in the pond were probably competing with the fish for what was available (and so were the nitrifying bacteria
), and the pond was stocked to capacity so there was no margin for error. The oxygen in the water was getting used up faster than it could be replenished, and the fish were desperate. The temporary solution was to agitate the surface of the water to get oxygen into it faster, in much the same way that you’d stir your coffee if you wanted to get the sugar to dissolve more quickly. While my husband rushed out to get a submersible pump with a fountain head while I created some temporary agitation by spraying the surface with water from the hose. The results were remarkable: Within seconds, the fish stopped gasping and began to swim normally (although they still stayed close to the surface). After we installed the fountain, we never had another problem. However, having learned a few things since then, I now keep fewer fish in a much bigger pond equipped with both waterfall and fountain.
All of this leads to an obvious question: What’s the critical oxygen level for fish? But once again, there’s no easy answer. Goldfish, for instance, are very efficient at getting oxygen to their tissues, so they’re able to withstand low oxygen conditions for longer than some other species. So can gouramis, which evolved in the low-oxygen conditions of shallow, stagnant ponds in Southeast Asia and have what is known as a labyrinth organ. This is a sort of primitive lung that allows them to breathe surface air when they can’t get enough from the water. Generally speaking, however, large fish need more oxygen per hour than smaller fish, and faster-swimming fish use more oxygen than slower swimmers, while fry often need more oxygen than adult fish. A fish’s consumption of oxygen also increases after feeding—some studies have found it to be by as much as 50 percent—because of the energy demands of digestion and growth. However, the picture is further complicated by the fact that fish are cold blooded, so the speed of their metabolism corresponds to the temperature of the water they’re swimming around in. The problem is that the warmer the water gets, the more active the fish become, and the more oxygen they need to fuel their increased activity (their demand can double or triple with every 10-degree increase in temperature), but warm water also holds less oxygen.
There are a number of ways to help counter this, and that’s where aquarium aeration devices come in. There are two basic types: Those that infuse oxygen directly into the water (such as airstones and decorative bubble walls) and—even more effective—those that expand the surface area of the water to give the oxygen more entry points. That’s what I was doing when I sprayed my pond with the hose, for instance, because each droplet of water picked up a lot of oxygen as it cascaded through the air; that’s also part of the point of waterfalls and fountains. And in aquariums, certain types of filters
help aerate the water as well. These include hang-on-back filters and trickle filters. Some fish farms involved in high-intensity aquaculture sometimes use such means to hyperoxygenate the water, which allows them to increase stocking levels without increasing the amount of water in their systems. However, there are some indications that this can stress fish; in one paper reported at a World Aquaculture Society meeting in 2006, Swedish researchers found higher levels of the stress hormone cortisol in Atlantic salmon that had been raised in hyperoxygenated water. Too much oxygen in water can lead to the potentially lethal gas bubble disease, in which gas comes out of solution inside the fish, creating bubbles in its skin and around its eyes. (Excess nitrogen, however, is a far more common cause of this disease.)
All of this brings us back to the humble airstone and the question I asked at the outset of this column: Is it necessary? Probably not, if by “necessary” you mean that your fish would die without it. (If that’s the case, you’re way overstocked.) Yes, it adds oxygen to the water, but if that’s what you’re worried about, there are more effective ways to do it—with a trickle filter, for instance. On the other hand, can airstones and other bubblers serve a useful purpose? Yes, within limits. They do add some oxygen to the water, and the bubbles they create help to keep water moving within the tank; by strategically locating it in areas in which water circulation might be less—near the bottom, for instance—you’ll help to keep suspended particles circulating so that they can be sucked out of the water by the filter. So while I’m not going to bolt out to the pet shop to replace mine the way I would if, say, my filter quit working, I’ll probably pick one up eventually. Because when it comes to aquarium aeration, every little bit helps.
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