Rain-Related Fish Kills: Runoff Toxins, Treated Lumber & Prevention

Two total fish kills in 3 months, both after rain — something washes in. Not frogs, not oxygen. Chemical poisoning.

Top Suspects

• Lawn/garden chemicals: Fertilizer (ammonia spike), herbicides, insecticides — lethal at ppm.
• Treated lumber: Arsenic, copper, creosote leach from railroad ties, deck posts, edging — especially when wet.
• Roof runoff: Asphalt shingle toxins, bird droppings, algaeicide treatments.
• Neighbor drift: Spray drift from adjacent property.

Investigation Steps

1. Map runoff paths: Where does water flow from? Driveway, lawn, roof, beds?
2. Check pond perimeter: Any treated wood touching liner or in splash zone?
3. Test after rain: Ammonia, nitrite, pH, copper (test strips).
4. Divert first flush: Trench or pipe to redirect initial 10–15 min of runoff.

Prevention

• Buffer zone: 10+ ft unfertilized, chemical-free border around pond.
• Remove treated wood near water — replace with cedar, stone, recycled plastic.
• Rain garden upslope — filters runoff before pond.
• Carbon in filter — adsorbs organics, some toxins.

Rain doesn't kill fish — what rain carries does. Find the source. Your fish depend on it.