Q. I have been keeping fish for more than 20 years: tropical, marine — you name it, I've had it. Goldfish have always been at the top of my list, and yet I've never had any lasting success with them. After reading several articles and columns of yours concerning stocking levels, I finally decided to try your advice. What really sold me on your advice was the way you said in not so many words, "You're killing your fish."

I am now relocating out of the country and have made many phone calls trying to find a good home for my fish. It seems most people I talk with think you are too extreme in your advice, especially on your suggested (very low) stocking levels. So I took the question where it mattered most. I looked into my 180 gallon tank with six lionheads, and looking back at me were six large, very healthy lionheads. I sincerely thank you and will continue to take your advice.

A. Thank you for your heartening letter. The true reward for writing these columns is the success stories such as yours. I have no doubt that most people you speak with will tell you my stocking recommendations for goldfish and koi are all wet. For the most part I believe this reflects their honest beliefs — no matter how misguided.

The source of the misinformation is usually some unfortunate rule of thumb, such as so many inches of fish per gallon of water, found in many tropical fish books. Or sometimes it's one of the few less-than-honest dealers just wanting to sell more fish.

I can only offer two observations. People who follow my advice and significantly reduce fish loads always report healthier fish that live longer and are more active. They also report far fewer disease and parasite problems. Often they observe spawning for the first time in their fishkeeping experience.

Conversely, I do not know of a single case in which increasing the fish load increased fish health or longevity. Indeed, in almost all cases disease problems multiplied quickly and ultimately the fish died.

So when I suggest that each goldfish should have 30 gallons of water, there's a very good reason. It's advice that has stood the test of experience. Thanks for writing.

A Concrete Idea
Q. I have been in the fishkeeping hobby for several years now, having had more than my fair share of luck and a few setbacks too. Having kept tanks that range in volume from 35 to 150 gallons, I now feel ready to move outdoors with my fishkeeping. Over the last year I have designed a pond that will be 12 feet long, 8 feet wide and 4 feet deep. The pond will be filtered by a trickle filter (2 feet square by 4 feet deep). The pond will be constructed of poured concrete and brick (I know most ponds use liners, but that's not for me). The only thing that is holding up progress is difficulty in finding something that is inert and safe for fish to coat the inside of the pond walls. I live in Canada and realize that product names may be different, but any suggestions would be greatly appreciated.

A. Concrete ponds can be "cured" and made safe for fish without any special coatings. Keep the newly constructed pond filled for a week, then drain and refill. After two weeks, drain and refill again. After three weeks, drain and refill again. Wait one additional week and drain and refill a fourth time.

The process is most effective if your water tends to be acidic: a pH under 6.5 or so. You can also aid the process by using standard swimming pool chemicals (such as sodium bisulfate) to lower the pH to below 6. In either case, the water changes are still necessary.

After this break-in period you can run a test. Wait a week after the final pond refill and then test the pH. Then place some fresh tap water in a bowl and let it sit overnight to allow oxygen and carbon dioxide levels to stabilize. Check the pH the following morning. If the reading is the same or within 0.2 units of the pond water, you can try a live fish test.

Put four to six feeder goldfish in the pond, taking care to equalize water temperature and pH differences between the pond water and their original water. Watch them carefully for several weeks. If they remain hardy and active, eat vigorously and do not exhibit strange behaviors, such as darting, listing or hovering, then the pond is ready for other fish.

However, there are two basic problems with this approach. First, it takes a long time. All totaled the test period is 10 weeks — virtually the entire summer in the northern half of the U.S. and in Canada. The second problem is that it may take much longer and many additional water changes depending on numerous variables. One never knows what will happen until it has happened.

Much of the waiting and uncertainty can be eliminated by using commercial, fish-safe pond sealers. These products might seem expensive — and it takes a lot of sealer to cover a large pond — but they are worth it.

A gallon of HERCO Liquid Neoprene, for example, runs about $80 (U.S.) and covers about 80 square feet in three coats. In general, a good seal usually requires at least three coats, so do not skimp. Your pond will need 3 to 4 gallons. I suggest buying extra because nothing is more frustrating than running out of seal the weekend you intend to fill the pond.

There are a variety of rubber- and enamel-based products also available. Again, in gallon sizes they cover about 70 to 80 square feet in three coats. These are sold by water garden suppliers under local brand names. If you cannot find any in your area try calling Waterford Gardens in New Jersey, at (201) 327-0337, or Sherer Water Gardens in New York, at (516) 261-7432.

I would not risk the possibility of poisoning the pond (the fish) by using ordinary swimming pool sealing paints. There is no information about toxicity to fish, and some of them do incorporate toxic chemicals to inhibit algae and mildew.

Goldfish as Bird Food
Q. The pond I built last summer is located in a marshy area in the woods behind my house. It is 8 feet wide, 6 feet long and 2 feet deep in the deepest area. There are rocks along the bank to walk on. This spring natural plants, such as skunk cabbage, began to appear.

I stocked the pond with three 5-inch goldfish, 13 2-inch goldfish and one 5-inch koi. I also stocked a few small bullfrogs and leopard frogs that I caught in a nearby stream. The fish are doing well, but there are a few things that concern me.

One is the great blue heron that comes by once in awhile. Because the goldfish are so easily seen, how can I keep the heron away? Also, is there a chance the goldfish will breed? Finally, is a diet of white bread good for my fish? I would buy food in the pet stores, but it is very expensive.

A. It sounds like you have a really interesting pond. You are right about the goldfish being easy pickings for the heron. There is really no reliable way to keep the heron away other than putting a net over the pond. Great blues fish during daylight hours, so a net might do the trick. If you had dug the pond to a depth of 3 or 4 feet, with steep sides, the heron would probably just stand on the bank and stare, but would be frustrated in its effort to catch fish.

Try landscaping around the pond bank to make it more difficult for the heron to approach the pond edge. A good 3-foot fence will do, although it may not look so great. Many devices are sold to scare herons — water cannons, barking dog tapes and so on, but none really works well over the long run. (One possible solution is to push stakes into the ground around the pond, leaving about 6 inches of stake above the ground. Use monofilament fishing line to connect all the stakes. The herons apparently won't step over the line. — Ed.)

You can be quite sure that the goldfish will breed, and in this context I should point out that your pond will quickly become overpopulated. Perhaps the heron is doing you a favor by keeping your goldfish population in check. Of course, the heron will almost certainly catch the most colorful and fancy of your goldfish first, largely because fancy goldfish swim the slowest and are easiest to catch.

A diet of bread and water is even more unhealthy for fish than it is for humans. Healthy goldfish need a variety of vitamins, minerals, oils, fats, proteins and other things that white bread — or any bread for that matter — do not contain. If you want to raise ornamental goldfish, one of the accepted costs is quality food. You do not have to use the premium brand fish foods. There are many good pond fish foods available at very reasonable prices. A good pelleted catfish food, for example, will also work fine.

Finally, by now you have probably discovered that the frogs you transplanted have wandered off. It does little good to bring them to the pond because they will just leave. Wait a season or two and frogs will show up on their own — and stay. By the way, you do not want bullfrogs — they will each your fish, too!

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 will 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 and 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.

Is It Bored?
Q. I have a bubble-eye goldfish that I keep in a hexagon tank. The fish seems to think that the sides of the tank are mirrors. It just stares at itself all day long. I figure it must be bored. What fish do you recommend to keep it company?

A. Your goldfish may indeed be bored, especially if the tank lacks plants for the animal to prowl around. Goldfish are gregarious animals, so a tankmate or two could make life more enjoyable. (I am anthropomorphizing here — I really have no idea about goldfish emotions.)

The short answer to your question is: The best companion for a goldfish is another goldfish. In this instance you would want to stick to the same variety — that is, a bubble-eye.

Bubble-eyes are very poor competitors for food in a tank with other goldfish lacking a similar "handicap." If you add a more vigorous goldfish variety the bubble-eye will suffer.

That said, I would not encourage you to get another fish if you are going to keep the hex tank. Unless it is well over 30 gallons another fish would add too much biological load to the tank.

Instead, you might consider adding a substantial crop of natural-looking plastic plants. Yes, plastic! The newer products are amazingly lifelike and will give your fish something to fool around with. Within a month microscopic algae will be growing on the plastic leaf surfaces, removing ammonia from the water just like live plants do — but more efficiently. Your goldfish will be able to graze happily and explore the aquascaping.

Line the Liner
Q. We have a koi pond with a PVC liner. It has been in the ground for about six years and has now cracked along the edges, and seems to be getting some splits in it. We plan to replace it next spring. My question is simple: Should I leave the old liner in place and just put the new one over it, or should I remove the old one first?

A. Leave the old liner in place. It will protect the new liner by providing some cushioning below. However, you should perforate or slit the old liner before setting the new one on top of it. Puncturing the old liner will allow any rain water that runs between the two to seep into the ground.