Can I turn my swimming pool into a Fish pond?

Q.Do you have any experience or thoughts on using swimming pools or kiddie pools for a fish pond? I have seen snap together and blow up children’s pools that will hold 500 gallons or more. What problems would I encounter if I tried to use a pool as a fish pond?

A. I have used all sorts of containers as fish ponds, including children’s pools. In theory, anything that can reliably hold water can be used to keep fish. The one major caution to consider is material toxicity.

Does the potential fish pond’s material contain compounds that are harmful to fish? For example, many manufacturers of inflatable vinyl pools incorporate algaecides, stabilizers, ultraviolet inhibitors, or other chemicals into the material to improve product durability and longevity. This is valuable in products intended for human use but possibly fatal for fish. These chemicals may leach into the pond water during the first season of use, with toxic effects.

Hard plastic wading pools, may also contain harmful compounds, although I have found this to be the case less often than with soft vinyl pools. Unfortunately, there is really no way to be certain prior to purchase.

The best thing to do is to assume that the pool manufacturer has incorporated some harmful substances in the pool. You will want to “cure” the pool before putting in fish. Curing can be accomplished in a number of ways. You can simply fill and empty the pool every couple of days for a few weeks, leaving the pool out in full sun. Sunlight helps to speed the chemical reactions along. (Avoid full sunlight, which can overheat the water, when there are fish in the pool.)

You can also add potassium permanganate to the water at a concentration of 50 parts per million. Replenish the chemical for the first two or three fillings.

When the treatment period is over, refill the pool and let the water sit for several days. Then add a few inexpensive fish, some snails, and large clumps of aquatic plants. Watch them for a full month. If all seems fine after a month, the pool is safe for use as a fish pond.

Another concern is how will you protect the pond’s inhabitants from predators? These large open water containers, placed so conveniently on the ground, are magnets for dogs, cats, wildlife, and children. Perhaps some type of netting cover makes sense. After all, many aquarists find that tank covers are a necessity if for no other reason than fish can jump out of those nice aquatic homes we provide. So a cover or barrier of some sort is advisable. If you buy a deep wading pool, you can just partially fill it and let the remaining depth act as a barrier.

You will also have to work out some schemes for filtration, aeration, and temperature control. Canister filters will do a great job, and there are many fish pool filters to choose from. Remember, mechanical and biological filtration is essential unless you have very few fish in a very large volume of water. The filter return can provide aeration.

If the aquarium pool is going outside, do not place it in full sun. These shallow pools have very large surface areas per unit volume of water, and the water will quickly become much too warm for the fish to survive in.

The exception would be pools with dense plantings of water lilies. In this instance, the floating leaves will shade the water. Nevertheless, you need to be very careful with pool placement, and I suggest monitoring mid-day water temperatures anyway.