Filter Current Question

fsih need to rest overnight, they prefer to have some quiet place in the tank.

I don't know if it will kill them but if they are weak from no rest they will probably get sick, that could kill them.
 
How big is the tank and what kind of fish do you have/want? Some fish prefer a strong current and some can't stand one! But that in its self should not kill a fish. Most filters I think you can adjust the flow on so that it is not as strong too.
 
As Holly said, different species prefer diffrent conditions. It's possible to exhaust fish, especially fry, with too much current - my diatom filter came with a warning stating that while it can be installed and used 24/7 this isn't advised due to the very strong current it creates and the possiblity of exhausting the fish.
 
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