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Limestone: Fact or Fiction...
Have you ever had anyone tell you that you cannot put limestone in a pond? Have you ever wondered if it is true or not? Although it's generally true, there are some instances when limestone is very hard and dense, or weathered to the point that it is safe to use. Moss or lichen growing on a stone is a good indication of this.
Normally, the pH is raised because the limestone seeps minerals into the pond water, raising the hardness and eventually driving up the pH. These minerals, namely phosphorous, along with several other micro nutrients, are responsible for increasing the growth of algae. But, when stone has weathered to a point where los pH-loving plants such as moss can grow, it no longer changes the pH of the water.
There is another problem that comes with very high pH levels - your fish can become irritated and potentially stressed. This is really only a problem when the pH begins to climb much above 9. This may sound really high, but keep in mind that there are countless ponds across the country with a high pH and very happy fish. My own pond runs at 8.7 pH all the time.
When in doubt, the way to tell if a stone is safe to use is to pour a little vinegar on the stone in question. If it foams and bubbles profusely, it is a type of limestone that you probably don't want to use. However, if there is only a slight, slow bubbling, then you are dealing with a type of limestone that could possibly be used in a pond. If there is no bubbling at all, it isn't limestone.