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I've heard a lot about it.

is it dangerous for my turtles?
and what exactly is it?

from what I've heard, it is when a new owner puts all the fish he wants for the tank the moment he gets it, when the tank is brand new, filter is new, and the water is new. Basically is it when the water and filter havent built up the benefitial bacteria needed for proper filtering and water conditions. Without proper water conditions, fish are more vunerable and often die. Not too sure about turtles though, I would assume as long as you put water conditioner and declorinator in the water, they should be perfectly fine while the filter builds up the bacteria cultures and the water stabilizes out.

ohhh okay! thank you

"New Tank Syndrome" is when ammonia builds up in the water before the nitrifying bacteria get a chance to develop and remove it. The water frequently gets cloudy and even smelly. It's highly toxic to fish, but not as bad for turtles. Amonnia can irritate thier skin and eyes if there's too much in the water, so until the bio filter gets going you should do frequent partial water changes (with dechlorinated water) to dilute the ammonia.