Q. I have two questions. First, do fish grow faster in a big tank? Second, I have four goldfish in a 10-gallon (38-liter) aquarium and they have not noticeably grown in several months. I feed them one to four times per day. They eat flake food, but refuse shrimp pellets. What should I feed them to make them grow faster?

A. Fish may or may not grow faster in a big tank or pond depending upon the fish load. Too many fish in a given volume of water results in unhealthy water conditions. The fish feel stressed and so they do not eat well. As a result, much of the energy that might go into growth is wasted.

Four fish in a 10-gallon tank would probably be healthier and grow more normally if moved to a 50-gallon (189-liter) tank. However, if you then add another 16 fish to that 50-gallon aquarium, you are back where you started, or at least the fish are.

The same problem arises when two small goldfish are kept in a 10-gallon aquarium. At first they do very well. They grow full-bodied and show good color. But at some point growth slows or ceases, colors fade and the fish become lethargic. Long before they reached this point the fish had already outgrown their tank. In other words, fish load does not only refer to the number of fish per volume of water but also the size (mass) of the fish.

The best way to ensure normal, healthy, robust growth is to keep the fish load low. To me, that means one goldfish per 20 gallons — maximum! Two in 55 gallons is even better.

I always wonder why hobbyists want their fish to grow rapidly. Studies on fish and other animals have shown that accelerated growth is strongly correlated with premature aging and death. In contrast, fish that receive slightly less food than they would like — that is, they are always actively searching for food — grow slower but live healthier and longer lives.

Goldfish will eat just about anything, but they are actually vegetarians. Shrimp pellets are just not their thing. You would be better off serving parboiled frozen peas with the shells removed. Place some frozen peas in a dish with water and microwave for a minute or two. Remove the shells and serve.

Also, remember that these fish are grazers. Several small meals per day are better than one big feeding.

Not Copper
Q. I'm an electrical engineer who has been keeping tropical fish as a hobby for years. About 10 years ago I built a 2000- gallon pond in my backyard in which I keep koi and goldfish. The algae problem in the pond is horrendous no matter what kind of filter I use.

Recently, a fellow engineer asked me to design an electronic filter for swimming pools in which a 98.5-percent copper and 1.5-percent silver electrode is used with a pulsating DC current to saturate the pool with copper ions to kill germs and algae. It produces sparkling water. The ion level is 2 to 4 ppm when it is operating properly.

My question is: Can I use this type of filter in my pond without hurting the fish? On the basis of what I have read about using copper-based medications, I know that copper at some level is harmful to fish. But at what level? I am very reluctant to try this device without more information.

A. Well, 2 to 4 ppm of copper in your pond water will certainly eliminate algae — and fish, too. Your hesitation at trying this gizmo out on your pond (assuming you did wait for my reply) saved your fish an agonizing death.

The exact concentration of copper that is lethal to fish depends on many variables: the fish species, the type and concentration of dissolved solids in the water and so on. In general, at the concentration you are considering, about half of the fish might die within a week — the rest would follow shortly thereafter. As a point of reference, it is generally recommended that ambient copper levels in ponds be kept below 0.015 ppm in order to minimize chronic stress to the fish — this is less than 1/100th of what you are considering.

If you want to use technology, the simplest and most effective way to eliminate the algae is to use an ultraviolet (UV) sterilizer. These always work if you size the unit correctly. You should choose a unit that delivers UV radiation of at least 30,000 microwatt-seconds per square centimeter at the flow rate of your pond system. Because most of the commercial units sold are calibrated for 15,000 microwatt-seconds per square centimeter at maximum rated flow, you should buy a unit that is rated for twice the maximum flow rate of your pond's filter system in order to achieve the correct level of radiation.

A typical unit for a 2000-gallon pond that circulates one complete volume per hour would require two 30-watt bulbs. It would consume about 3 kilowatt-hours per day of electricity. Depending on your utility costs, this might add between $4.50 and $9.00 to your monthly electricity bill.

A unit of this size is fairly expensive — perhaps $500 or so. Be sure to buy a unit with quartz sleeves to shield the bulbs. The quartz sleeves protect the bulbs from direct contact with the water — which is necessary for water temperatures below 72 degrees Fahrenheit (22 degrees Celsius) — without interfering with the UV radiation. If you try to use the unit without the sleeves you will probably destroy it in short order.

The UV sterilizer is utterly harmless to your fish. For many years some hobbyists argued that these units would have a weakening effect on fish immune systems. This false notion was based on the mistaken belief that if the fish lived in sterilized water, it would never face "challenges" to its immune system and therefore would be unable to handle any disease-causing organisms in unsterilized water. In fact, UV sterilizers don't even come close to truly sterilizing the water.

Indeed, even at the increased irradiation level I suggest these units will not kill most pond parasites. To kill ich parasites or vulnerable anchorworm life stages, for example, you would probably want to increase the exposure level five to 10 times! And even if you did that, the recirculation rate in most ponds — averaging less than one complete turnover per hour — is far too low to expose most pathogens to the sterilizer.