Small Goldfish Pond

Q. I have been a goldfish lover for about 10 years. I currently have six goldfish in a 10-gallon tank. I use an undergravel filter in the aquarium. Is there a filter that is better suited for goldfish, and if so, will it help rid my tank of algae? Are there other fish that can live with goldfish? I have tried to research these questions, but with little success.

A. In keeping with my standard sermon, I have to begin my response by saying that if you are a true goldfish lover, you should either 1) get one or more larger tanks, or 2) dig a garden pond to house your pets. A 10-gallon tank is really not adequate for keeping goldfish healthy for more than a short time.

An undergravel filter is a combination mechanical and biological filter (but is intended for use only as a biological filter). By pulling water through the gravel bed, suspended particles are screened from the water and held among the gravel pieces. Only some of this solid material is broken down by bacterial action. Moreover, the bacteria themselves tend to gunk up the gravel bed in direct proportion to the amount of solid material drawn into the bed. Not surprisingly, over time the gravel bed clogs — which is, after all, what we expect a mechanical filter to do.

Now, if the fish load is very light (much less than I have ever seen in any hobbyist's tank) the bed may not clog at all. But under normal conditions regular cleaning is required. The amount of solids that goldfish produce is much greater than most tropical fish because of their large size and voracious appetites. Therefore, undergravel filters in goldfish tanks tend to clog more often and more severely.

This is not an argument against undergravel filters, but a caution that in goldfish tanks undergravel filters tend to demand more maintenance. If you don't want to provide this maintenance, then an undergravel filter is not a good choice.

As noted above, an undergravel filter also functions as a biological filter, providing a home for nitrifying bacteria to detoxify ammonia. And, as long as well-oxygenated water can flow through the gravel, this setup works fine. However, because of the high waste loads produced by goldfish, maintaining this flow of oxygen-laden water through the gravel bed becomes a problem. Thus, undergravel filters in goldfish tanks bear close watching and demand regular cleaning.

Are their "better" solutions? There are certainly alternatives. I believe that an external power filter is a more practical mechanical filter, and is easier to maintain. In fact, if you maintain what I consider to be a healthy (low) fish load in an aquarium, no other filtration will be necessary. In other words, the nitrifying bacteria on the surface of the gravel bed will be more than enough to handle the ammonia, without the need to pump water through the gravel. This is how all of my goldfish tanks are set up.

A sponge filter also does a great job as a biological filter, and as a mechanical filter too. As with the undergravel filter, solid matter will eventually clog the sponge and significantly reduce the biofiltration.

Some goldfish breeders claim that undergravel filters used in tanks with long-tailed goldfish lead to frayed tail ends. This is allegedly a consequence of the downward flow causing the fragile tail lobes to drag along the gravel. This may well be true, but I cannot say so from first-hand observation.

Because goldfish tend to be peaceful scavengers, they should get along with almost any fish as long as the tank is not too crowded. But, remember, goldfish are coolwater fish — not tropical fish. Tankmates should also be comfortable in an unheated tank kept at room temperature. Frankly, I would not mix goldfish with other species because goldfish — fancy goldfish, in particular — are ungainly mutants that do not compete well with fish that retain their natural forms.

Algae problems are caused by a combination of excess light entering the tank and excess nutrients in the water. The solution to the former is shading your tank from sunlight or cutting the duration of artificial illumination substantially.

Excess nutrients are most likely the result of overfeeding. Healthy ornamental fish should always be hungry — but they should not be fed to satisfy that hunger. Try cutting back feedings by 50 percent. Animal studies show that fish that are underfed live longer and healthier lives.

The Drained Pond
Q. I have a 1200-gallon pond that I built two years ago. I have never drained it for cleaning. I just remove leaves and junk as they accumulate. A friend told me that the pond should be drained and cleaned each year to prevent parasites from infesting my fish. How often should a pond be cleaned?

A. I think your friend may have gotten his tip from aquaculture practices. Intensive fish farming imposes management practices that are not necessary in ornamental pondkeeping.

Fish farms — including ornamental fish breeding facilities — may drain ponds on an annual or biannual schedule and either treat them or allow them to remain dry for a year as a method for controlling fish pathogens. This is a necessity for these operations, given the very high quantities of fish that move through breeding ponds and the high densities maintained. But the practice would not have much practical effect in hobbyist ponds, with one exception.

The exception is severe and persistent infestations of anchorworms or fish lice. This would justify draining and cleaning the pond.

If you carry out regular partial water changes and do basic debris and leaf removal, there is little to be gained by draining and cleaning. You will not change pathogenic bacteria or other fish parasite populations substantially, but you might inadvertently stimulate undesirable algae growth (i.e., planktonic algae or long-filament algae) by removing short filament algae on the pond sides. You may also run the risk of damaging the pond or injuring the fish.

Intensive cleaning will also significantly inhibit invertebrate populations in the pond, which will limit its natural quality. All in all, there is little reason to worry about performing this kind of maintenance on a backyard pond.

Absolutely Biological
Q. I am building a 2000-gallon pond. I plan to keep a dozen koi or so and am considering what kind of filter I need. I was thinking about using only a plant filter, but I was told a biological filter was an absolute requirement. Is this true?

A. The term "biological filter" is used by aquarists and pondkeepers to connote a filter specifically designed to provide a good habitat for nitrifying bacteria that detoxify ammonia. Beyond that, the term has no direct physical meaning.

A good plant filter design can make an excellent biological filter in two respects. First, the plant roots and submerged stems make an excellent substrate on which nitrifying bacteria can grow. Second, the plants themselves will remove ammonia from the water and hence act to biologically filter the water as well. Therefore, if you design the plant filter properly, you will have all the ammonia removing capacity you need.

A plant filter also makes an excellent, as well as maintenance-free mechanical filter. I would recommend a plant filter box that holds about 200 gallons of water and is about 1 foot deep. I would not use a gravel or dirt substrate, but instead place yellow iris (Iris pseudocarus) bare root in the water with some stones to hold them in place. Alternatively, floating water hyacinth will work well. In either case, the box should be filled with plants spaced 6 inches apart. Recirculate the water through the plant filter at least once every two hours.

Never, ever use plant fertilizers. The nutrients will only cause problems and they aren't needed.