Koi Mass Death After Roundup Near 200,000 L Dam

200,000 L dam. 80+ koi (>40 cm) dead in 1–2 days. Sprayed Roundup (glyphosate) 1.5 L/100 L dilution, ~100 mL near dam. Rain 2 days later. Fish died 5 days post-spray. Asks: glyphosate cause? When safe to restock?

Glyphosate Unlikely Direct Killer at That Dilution

100 mL of 1.5% solution in 200,000 L = ~1:500,000. Sub-lethal. But glyphosate pH ~2.0—could crash pond pH if KH low.

More Likely Scenario

pH crash → immune suppression → latent bacterial outbreak (Aeromonas). Check dead fish: hard body, no slime = bacterial septicemia.

Restocking Timeline

• If pH crash: wait until pH/KH stable 2+ weeks.
• If bacterial: disinfect? Impossible at 200,000 L. Wait 4–6 weeks, test water, add sentinel fish.
• Glyphosate half-life: days to weeks. Bioassay with test fish before restocking.

Test water NOW: pH, KH, ammonia, nitrite. History suggests secondary infection, not direct poisoning.