What is the best type of cement to use when repairing a leak in a fish pond? I don't want to hurt the fish ! It is a it is a stone fishpond

We would need more information about the repair to determine the most suitable product for the application. For example, you may consider using QUIKRETE® Hydraulic Water-Stop Cement (No. 1126), which is designed to be mixed to a heavy putty consistency for use in filling active leaks in small cracks or holes in concrete and masonry – a golf ball size mix is appropriate for most applications. Please note that QUIKRETE® Hydraulic Water-Stop Cement is not meant to be used as a coating and not to be used for large areas (must be less than 2 square feet). Plese note that the repair area must be clean, unsealed/uncoated, sound, free of loose debris, dirt, dust, and any other potential bond breakers. When making repairs, getting a good color match can be difficult – we always recommend trialing the repair material in a small, inconspicuous area first and waiting 24 hours to ensure that the appearance, including color, is acceptable. Cementitious products such as QUIKRETE® Hydraulic Water-Stop Cement will increase the pH level of the water in the pond during first month of curing. After placing the concrete, wait 7 days, fill fish pond with water only, wait 1 day then check the pH. Then wait one additional day and recheck the pH to determine if there was any change. If pH results indicate that it is safe to do so you may add the fish and vegetation. The pH will continue to be affected to some degree by the cementitious material which is still curing. You may call us directly at the QUIKRETE® Technical Center at 1-800-282-5828 if you have any further questions.