Limestone is a name used for rock that may be considered for pond edging or other parts of a feature. Whether a particular stone affects pond water depends on the material, how much of it contacts the water, and the conditions already present in the pond. Looking at a rock is not enough to determine its effect or decide what is suitable for fish.
Start with the stone and the whole pond
Ask what rock is being supplied and where it will sit: above the waterline, along an edge, or in regular contact with pond water. Consider the amount of rock, the water source, and any existing concerns about water conditions. A stone described as weathered or natural is not automatically suitable for every pond, and the appearance of a freshly cut surface does not provide a complete water-quality assessment.
What can observation tell you?
Note when the rock was added, whether the pond's appearance or fish behavior has changed, and whether the water source or maintenance routine changed at the same time. These observations can help identify a pattern, but they cannot prove that limestone caused it. If conditions are concerning, test the water with an appropriate method and record results over time rather than relying on color or clarity alone.
When should you seek a site-specific assessment?
Before adding a substantial amount of unfamiliar stone, or if a pond changes after rockwork, ask a qualified pond professional to assess the materials and water conditions together. The appropriate decision may differ between an ornamental feature and a fish pond. Follow the liner and feature instructions for any rock placement, and do not add a water treatment based on a guess about the stone.
Keep the question open until you have evidence
Limestone is not a single yes-or-no answer for all ponds. Identify the actual material, understand where it contacts the water, and use relevant testing and professional advice to guide a decision for your feature. This article does not set chemistry targets or certify a rock as safe.