mass fish death

Question
QUESTION: Hi David, Last weekend I bought some discount fish from Petco. Since then we have had a mass death in the tank. I have a 48 gallon tank w/plants and plastic and all was well and happy. I've lost 20 fish this week, They don't have ick, nothing looks out of sorts other than every morning I get up to see what has died overnite. The fish when they die, just look like they are dieiing all of a sudden. What the heck is going on? Any ideas or suggestions

ANSWER: Hi Joyce:  You probably have a toxic tank... either the water chemistry has spiked or some other chemical agent such as fly spray, furniture polish, etc has gotten in the tank... airborne chemicals kill fish really quickly.  Change about 60% of the water and add more beneficial bacteria to the tank.  You may want to change the carbon filters as well.  keep me posted... dave

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QUESTION: More fish died today - 3. The water tested fine for ph, nitrates, nitrites etc. My husband is anal about the water. Interesting about the spray..I used carpet cleaner around the tank on the day they started dying..you don't think something got in the tank? I hope the cleaning lady didn't spray glass cleaner on the tank ... would that kill the fish too? Whatever it is I you're right about the toxic water...I have the death tank right now and it's so disheartening. My husband put some medicine for a bacterial thing in the tank today. Did another 1/4 change of water and took out the castles etc. We'll see what happens. Should I do a complete water change if some sort of chemical got in?

Answer
Hi Joyce: Sorry to hear of your continual fish loss.  I am not a chemist so I cant answer your question about the carpet cleaner.  Household chemicals like aerosols can change the pH of tank water with devastating results.  Aquarium testing kits only test for the basic water chemistry so in situations like yours I would do a minimum of 50% water change especially since you are still losing fish.  A disease would show sign and symptoms and would effect at most one type of fish maybe two... but when you have many fish dying and still looking like they should be alive... that points to some kind of poison.  dave