Cement is a safe material to use in fish ponds, but it does need some curing and cleaning before you fill the pond and add fish. In the first few weeks of exposure to water, it can affect the pH balance and make it difficult to keep at a level fish prefer. Simply delay your plans to fill and use the pond until after you get a chance to drain it, let it stand for two weeks, and then repeat again. This should take about a month and give you a cement-lined pond you can trust not to harm your fish.
Watch out for acrylics and other additives sometimes blended into cement to make it more flexible and water resistant. While these products are recommended for foundations and other construction methods, they’re not a great match for ponds. Most aren’t tested to see if they’re fish and plant safe or not. Don’t gamble with the health of your fish. Install a flexible liner before pouring cement to keep water from seeping out without any additional coating or sealant on the cement. This reduces the chances of leaching that could affect water quality in the long run.
