Insecticides and Pesticides: The Deadliest Aquarium Contaminants
Insecticides are designed to kill arthropods, so they are lethal to shrimp and fish at trace levels. Learn the routes (flea meds, sprays), the emergency response, and prevention.
Insecticides are the single most dangerous class of accidental aquarium contaminant. They are designed to kill arthropods, and shrimp, crayfish and crabs are arthropods, so these chemicals are extraordinarily toxic to aquarium invertebrates and very toxic to fish at concentrations far too low to see, smell or test for at home. Inverts usually die first, and a contaminated tank can suffer rapid mass mortality.
The main active ingredients
- Pyrethrins and pyrethroids (e.g. permethrin): jam open the sodium channels in nerves; highly toxic to fish and aquatic invertebrates
- Fipronil: extremely toxic to crustaceans, with a reported 96-hour LC50 for brown shrimp of just 0.12 micrograms per litre
- Imidacloprid and other neonicotinoids: a nicotine-based neurotoxin dangerous to aquatic invertebrates that can persist for a 70-100 day half-life in acidic water
- Organophosphates: nerve agents affecting acetylcholinesterase
How it gets into the tank
- Spray or aerosol bug killers and foggers/'bug bombs' used in or near the fish room
- Ant and roach gels, and garden pesticides drifting in through a window
- The classic route: handling the tank after applying a flea/tick spot-on (fipronil, imidacloprid, permethrin) to a dog or cat without washing, or a treated pet brushing past
- New plants from suppliers that used pesticides, if not rinsed and quarantined
Emergency response
If you suspect insecticide contamination, act immediately. Move surviving livestock to clean, dechlorinated, temperature-matched water in a separate container. Carry out very large, repeated water changes on the affected tank and run heavy activated carbon or a chemical-adsorbing pad, since carbon adsorbs many of these compounds. Even with fast action the prognosis is often poor for invertebrates, so prevention matters far more than cure.