Mortar in Waterfalls: Risks to Koi and How to Mitigate Them

Why Mortar Can Be Harmful

Mortar and cement leach calcium hydroxide, raising pH and hardness. In a closed recirculating system, this can push pH above 9, causing ammonia toxicity and gill damage. A small amount may stabilize; a large amount can kill fish.

Making Mortar Fish-Safe

1. Cure fully: Let mortar cure 28 days before introducing water.
2. Acid wash: Scrub cured surfaces with a 1:10 vinegar-water solution; rinse thoroughly. Repeat until foaming stops.
3. Leach repeatedly: Fill, circulate, drain, refill—multiple cycles over weeks.
4. Test pH: Before adding fish, ensure pH is 7.0–8.0 and stable for several days.

Safer Alternatives

Pond-safe sealants (epoxy, polyurethane, or rubberized coatings) eliminate leaching concerns. For a natural look, use dry-stacked stone with a hidden EPDM liner underneath the waterfall.