Q. I have been a subscriber to AFI for several years. My primary interest is tropical fish, but I've been reading the pond section, and this year decided to take the plunge and install a 2500-gallon garden pond. I will be using an EPDM liner and was told to secure the top edge with a thin concrete lip. I've gone through past issues of AFI, but haven't found any mention of using cement to finish the liner edge. Could you offer some tips?

A. My advice is to skip the concrete. Perhaps it makes sense in warm climates — perhaps. But in a cold area like upstate New York that type of finishing will not last a single winter. Frost heaves will snap it into fragments, and within a few years the concrete edge will be reduced to rubble.

The concrete lip also negates two of the great advantages of liner ponds. First, liner ponds are easy to install. Even very large liner ponds can be built with the aid of a few friends and without special equipment (or great expense). Cementing the liner in place is a big job, and messy as well. A cement lip also raises problems of pH and alkalinity control. The cement must be sealed to prevent mineral leaching into the water.

Second, liner ponds have the wonderful quality of being easy to modify and reshape at will. I've lost count of the number of times I've altered my ponds' designs over the past 10 years. Once you cement the liner edge in place, you can no longer modify the pond's layout. And believe me, no one is ever satisfied with their first attempt at laying out a garden pond (or their tenth, for that matter). I also imagine you will find the cement lip less than attractive.

Instead, I recommend using field stone to build a layered edge. It provides a beautiful finished look, but one that can be completely removed in half an hour. It can be rearranged to accommodate sunken pots, pond ornaments and so on, and it moves with the frozen ground, so the liner is spared any stress and there is no cracking or breakage.

For a more formal look you can use patio blocks. Because most blocks are made from cement they should be sealed. In either case use two or three courses of blocks to cover the liner. Fold the last 6 inches or so between the top and middle course.

Deathly Dropsy
Q. My favorite goldfish, Molly, is about a year old. She is a good-size red-and-white oranda. Molly has developed dropsy. I've spoken to several people, including the owner of the local aquarium store where I got her, and all have said to destroy her. I don't want to do that. Is there a cure? I found several products in the aquarium store that say they treat dropsy.

A. Advanced dropsy is a severe internal bacterial infection resulting in a generalized septicemia. That is, the bacterial infection and bacterial toxins permeate the bloodstream and move throughout the fish's body.

In goldfish the infection almost always begins in the intestinal tract, caused by the bacterium Aeromonas hydrophila, (other species, such as Pseudomonas, can cause similar problems). Bacterial toxins break down intestinal tissue, and fluids begin to fill the visceral cavity. Progressively, the bacteria destroy internal organs.

Chances are that by the time you notice a real problem — severe bloating and standing scales — the damage has been done. Nothing will help at this point. The only sensible course of action is to destroy the fish.

But, before you put the fish to sleep, make sure the problem really is dropsy and not some treatable problem. Often, the external signs we associate with dropsy — bloating, scales standing out from the body, balance problems — also appear under other circumstances. These false instances of dropsy account for the "miracle" cures reported in the hobbyist literature. They are really not curing an Aeromonas sp. bacterial infection. (Yes, if caught very early medicated foods can cure this bacterial problem. But, in almost all instances, hobbyists do not notice the problem soon enough.)

Some goldfish are just fat and odd-shaped. They look weird and swim weird. However, in fat goldfish, the scales do not stand out from the body as they do in cases where abnormally high visceral pressures push out the scales.

Some female goldfish can exhibit dropsy-like symptoms at breeding time. I have a female fantail who does this every year at breeding season. As her eggs ripen, her abdomen swells, scales stand out and she has great difficulty swimming. Once the eggs are expunged, however, she returns to normal in a few days.

I also have a beautiful female koi who exhibits the same signs at breeding time. The first time I saw this I resigned myself to euthanizing her sometime in the subsequent weeks. But, it was never necessary. That was seven years ago.

So, let's be sure the problem really is dropsy. In advanced cases the abdomen has no firmness. When you touch it with your finger it feels like a plastic bag full of water. Scales may fall away and blood will seep from the touch point or, equally likely, from the anal vent.

An internal tumor (or large tape worms) may cause similar bloating and standing out of scales. And, for the most part, there is nothing you can do about tumors either. Some tumors are slow growing, others expand rapidly. Destroying the fish is the most humane thing to do.

The only sure way to tell if the problem is intestinal worms is to actually see worm segments dangling from the anal vent. A veterinarian can provide treatment for internal worms.

All of this begs the question, how do you destroy the fish? There are many ways to do this, but the one I recommend is to put the fish in a container of water and stick the container in the freezer. I suggest that you do this when no other family members are around.

Them Traveling Plants
Q. Some unknown plants have established themselves around the edges of my ornamental pond, and even in the rockwork. Should I pull them? Will they harm water quality in my pond, or poison my goldfish?

A. "To remove or not to remove," that is the question. The answer depends on your own subjective sense of aesthetics. If you think the plants are ugly and ruin the appearance of the pond, pull 'em.

But, you may want to let them grow. These plants have found a niche in the ecology of your pond that they can fill easily and flourish. Obviously they are plants that are already established in your area.

Each year a huge crop of Bidens frondosa (Beggar's ticks) takes up residence in an artificial stream that feeds my tea house pond. I used to tear them out, and within a week my pond turned green. Once I noticed the effect, I left them alone. The physical and chemical characteristics of my pond water have never been better, and the native vegetation has attracted birds, butterflies and other critters.

Other local natives that have set up house around or in my pond include Impatiens capensis (jewelweed), which is great for treating poison ivy, Dryopteris thelypteris (marsh fern), which is very attractive, Sagittaria latifolia (arrowhead), which many people buy from aquatic nurseries, and Phytolacca americana (pokeweed), a plant with attractive, but poisonous, purple-black berries.

These native plants have a number of advantages over trying to establish commercial exotics. First, native plants are easier to establish and require less care and maintenance. If they are able to establish themselves, then they have saved you a lot of trouble. Chances are, if you leave them alone they will thrive.

Second, native plants tend to start growing earlier in the season and can have a lasting, positive effect on water quality. The Bidens sp. start growing in mid April. My Iris pseudocarus (originally an exotic that has been in New England since the 1700s) doesn't get rolling until a couple of weeks later. When I manage to acquire water hyacinths (which everyone likes for the flowers and their ability to take up nutrients) they cannot be put outside until mid June. And because the April-June period is crucial for getting a handle on algae bloom, it is clear which plants must be doing the work.

Third, native plants are far more likely to have food and shelter value to local wildlife and therefore will attract animals in greater numbers and variety. So, think carefully about your decision.