There’s a common misconception that concrete is a hazardous material to fish and is risky to add to a pond. However, the biggest challenge that comes from putting concrete blocks in water is a mild raising of the pH balance. This does matter when you’re raising delicate koi, but goldfish and other fish are more resistant to these changes. This is primarily a problem with freshly cured concrete blocks and newly poured concrete. Letting the concrete sit and age for a few weeks to a few months reduces its ability to affect the pH of the water.
Basic concrete blocks aren’t manufactured with any ingredients or materials that are particularly hazardous to the water or fish. If anything was added to the outside of the block while it was used for another purpose, something hazardous could leach out. Many pond owners are tempting to reuse older concrete blocks they have lying around to put them to good use and avoid the pH change from new concrete. This is okay if you know how the concrete was used, but it could be a problem if you pick up random blocks from a construction site. Some blocks are treated with sealants, paints, stains, and other products that could leach chemicals into the water and hurt the fish.
