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Should I use Lava Rock in a Koi Pond?

Although popular with pondkeepers, lava rock does have some potential problems.

By Stephen M. Meyer

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Q. I will be digging a small pond in my yard this spring. I want to put a few fancy goldfish in it and some plants.

My question concerns material to put around the edge of the liner. Here in Hawaii lava stone is plentiful. I have seen it used to landscape around ponds, to build waterfalls and as an edging. Is it safe to use lava rock in a fancy goldfish pond?

A. Lava rock as a pond aquascaping material has been a matter of disagreement among koi and goldfish pond keepers. I find that it works quite well, if you keep a few points in mind.

Those who caution against the use of lava rock in aquascaping where fish may come into contact with it make a strong case, calling attention to lava rock's sharp and rough surfaces. Active fish can and do scrape themselves badly on it. This is actually much more likely to happen along the edge of the water rather than on a submerged piece lying on the pond bottom. In particular, the thrashing and bashing of goldfish and koi spawning includes the female and chasing male slamming against the pond edge. Even rough concrete pond edging can result in pretty nasty gouges in fish. Parasites and bacterial infection are always a threat with open wounds.

Even more benign "flashing" — when the fish scrape themselves sideways along the pond floor — can permanently ruin the appearance and show quality value of a pond fish. Lost scales, frayed fins and wound scars are reminders of damage.

Nevertheless, lava rock can make a fine pondscaping material, and in Hawaii it does give a natural look. (A lava rock pond doesn't seem like it belongs in New England.)

I would feel most comfortable using lava rock in a pond with fish that I did not hope to show competitively. The simple loss of a scale can transform a top competitor into just another pond fish. Scale loss is hard enough to prevent in any pond. Lava rock just adds to the problem. If you are investing big money in a goldfish (hundreds of dollars is not impossible) I would think twice about lava rock for aquascaping.

I also feel much more comfortable using lava rock with goldfish — especially moors, fantails, celestials and the like. Lava rock with koi in the pond seems like asking for trouble. The latter are just too active and athletic, and get too large. (I know I will hear from lots of koi keepers in Hawaii with lava rock ponds. I am not saying it can't work, merely that the risks of serious injury to koi are greater than with goldfish.)

Fancy goldfish are slower, less active and far smaller than koi. They are much less likely to injure themselves and less likely to sustain several cuts.

However, algae and other forms of slimy biological material will, over time, coat the lava rock in contact with the water and soften its edges. In fact, the lava rock will act as a wonderful medium for nitrifying bacteria. Assuming you run a small pump for basic aeration and circulation, your entire pond will be an integrated biological filter. So there is a positive side here.

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