Rocks in the Pond

There are 2 camps on whether rocks in the pond is a good or bad thing. Some say they aid in filtering the water. Others say muck can accumulate between the rocks creating a poisonous environment. Personally I haven't found either view to be completely true. But for me the look of rocks is something that adds a great deal of enjoyment so they're a must.

I had loose rocks in the main pond for a year to see how that worked out. Fish were fine, muck did accumulate but no more than if the rocks weren't there. The next year I emptied the pond, removed the rocks and mortared them down. Now I get the great look of rock and a pond bottom that's as easy to clean as bare liner. Rock bottoms can also be walked on by you, dogs, maybe even horses, without worrying about putting a hole in the liner.


930 gal, 8' x 11', 18" deep. 4th in series of 6 connected ponds. Mortared river rock over liner.

After 2 years.

Rocks give an alpine lake look.

This picture is shows the bottom of another pond in about 4-1/2 feet of water. This pond had not been vacuumed in about 4 months so there is a layer of silt but the shapes of the rocks still come through. I used head size river rock which is very inexpensive in San Jose. When vacuumed the color of the rocks come through very nicely.

Mortaring down rocks is work but no special skills required. Wear latex gloves. Cement is harsh on the skin and over a few hours of working with it can make your hands very rough. A full day of work and it can start to eat a few small holes through your skin and they hurt. 60 pound bags of mortar can be purchased for about $7 (2003). Just add water, mix, shovel the mortar into the bottom of the pond on top of the liner about 3-4" deep and push the rocks into the mortar. Kind of smooth down the joints between the rocks with your finger. Remove mortar from the face of the rocks as you go using a brush and water, dish washing brush works good. Let the mortar cure for at least 10 days, 28 days if you can wait. Hose down the pond bottom and vacuum that water and debris with a shop vac.

You can acid wash the rocks and mortar with 10 to 50% muriatic acid in water using a garden sprayer. This can reduce the amount of lime released from the mortar into the water. Personally I would not bother with this step. I've done acid washes and also not done them. There was no difference in pond startup. When starting up any pond, mortar or not, pH can have big swings. Test and wait for stable pH before adding fish. I'd wait at least 4 weeks before adding fish, mortar or not.

You can also apply a concrete sealer. This is really a waste of time and money but doesn't hurt anything. It's the term "sealer" I think people like. I sealed the mortar in one small pond. Since I never saw any problems in my non-sealed ponds there wasn't any improvement.