Q. In our family/dining room I have a 30-gallon-high tank with a power filter and an undergravel filter and an airstone for aeration. Three 3-inch orandas were in this tank for 1½ years. I do a 20-percent water change each week.

In November I sprayed a pyrethrin-based flea spray on a dog crate that was situated about 12 feet from the tank. The spray had no contact with the tank — only fumes in the air. That evening the fish were acting a little dopey. Over the next two weeks one oranda lost its ability to stay level. It hung head down in the water, then later it was on its back. It did not float — just swam upside down. A few days later it died.

About a week after that I noticed that my red cap oranda had some scales standing out. In the days that followed this spread. I am now treating it in a 5-gallon hospital tank for dropsy.

My question is, could the flea spray have poisoned the fish and would it take this long for the fish to die? Nothing else has changed in the tank. I cannot figure out why two beautiful fish would die so suddenly. It's driving me crazy.

A. Well, there is a good possibility that the flea spray did cause the death of your fish. Pyrethrin is a natural organic pesticide derived from the dried flowers of Chrysanthemum cinerariaefolium. In fact, mixtures using pyrethrins have been tried as a method for killing anchor worm, but the toxicity of these solutions to fish eliminated any chances for their use.

There has not been much detailed research on the toxicity of this compound to fish, but from what information is available it seems to be lethal. Carp (fairly large fish) were affected by 1 part per million (ppm) and were paralyzed and died at 5 ppm. Guppies died within five hours of being exposed to 1 ppm. And 0.009 ppm was lethal to juvenile salmon.

Of course, in a flea powder there are likely to be chemical binders and enhancers that may increase the toxicity of pyrethrin even more when exposed to water. It is impossible to say how much got into the tank, but if you could smell it in the air then it is not hard to imagine that the pyrethrin dust could settle on the water surface where feeding goldfish would suck it in as they ate.

It might well affect nerve system functions (as limited testing shows), which would account for your fish's upside down swimming. It may also affect immune abilities, leading to dropsy and other bacterial diseases. In any case it is not surprising that it took several weeks for the fish to die. That is how sub-lethal doses kill fish: The animals are weakened and something else finishes them off. A full lethal dose would have killed them quickly.

If you have to use any insecticide, fungicide or similar products in the house, you must take precautions. First, cover the tank completely. Second, turn off all power filters and air pumps. Third, open windows and use fans to flow air away from the tanks.

When spraying is finished, add fresh aquarium-grade activated carbon to the power filter and restart it. Carbon will remove many pesticides from water. After 24 hours discard the carbon and replace it with fresh material.

Pondkeepers face this problem all the time when they (or their neighbors) choose to use toxic chemicals to control pests in the landscape near ponds. Accidents happen.

Stones and Boulders
Q. Later in the spring we will be building a pond along the side of our house. We are planning to use a liner and want to hide the liner edge beneath stones and boulders. Our property is littered with stones of all sizes and we expect that we will have more than enough after digging the pond.

We have two basic questions. First, how wide and how deep should the ledge to hold the stones be around the inside of the pond? Second, what is the correct way to place boulders around the inner ledge of the pond? I think the larger ones should go on the bottom, below the waterline, with the smaller ones on top. Janet wants the small ones on the bottom and the larger ones on top. I think her setup would be unstable.

A. This is one of those little pondscaping details that can make a big difference in the final appearance and operation of the pond. Of course, you never find even a mention of this in pond-building instructions.

Ledge depth should be at least 6 inches below the designed water level of the pond. In my experience, however, I have found that 12 inches of stone below the waterline is greatly superior. Not only does it look better and offer better protection for the liner, but most importantly, it provides substantial habitat for aquatic animals and bacteria (including nitrifiers).

The large number of nooks and crannies in a 12-inch course of stone present excellent habitat spaces for invertebrates of all kinds, amphibians and reptiles, and even small fish. Overall, you will find considerably more life in your pond with a 12-inch stone wall below the waterline.

The stone surfaces are an excellent substrate for nitrifying bacteria and periphyton algae (algae that attach to underwater rocks). An abundance of both can help eliminate ammonia, reduce the likelihood of planktonic algae blooms, and help maintain stable water quality. The periphyton algae and the invertebrates that live on it are also a good source of food for pond fish.

In general, the ledge width should roughly match the width of the largest stones you plan on using around the pond edge. You can always make the ledge slightly smaller if you want the larger stones to overhang. In either case, you will want to slant the ledge back toward the pond wall so that the stones will tilt backwards.

Now to the question of stone laying. I must admit that your approach — big on bottom, small on top — seems more logical in theory, and it was the way I did it for many years. It just turns out to be wrong.

After a half dozen very instructive visits to Japan I learned that the most aesthetically pleasing and structurally sound approach is to assemble the smaller stones along the ledge bottom and then stack the larger ones on top. Using smaller stones on the bottom spreads the weight of all stones more evenly. It also provides a larger number of contact points along the bottom. (When large stones are used on the bottom course they often rest on just a few points). Not only is this more stable, but it also reduces the chances of puncturing the liner. Then, too, the smaller stones on the bottom provide more small spaces for aquatic animals to live in.

Larger stones give a more natural look because many will be partially out of the water. A few very large boulders can be placed as platforms straddling the bank and the pond.

In your area of the country natural streams and ponds often have rocky banks. Go out and take a good look. Mimicking nature will give your pond a quality that will be hard to beat.