If I put tap water in first then water conditioner will the chlorine in my tap water kill all my beneficial bacteria?

[Alphabot]

Setting up a new tank and I put some rocks and filter media from my other tanks in to help cycle but I just realized that even though I've always done it this way does the chlorine kill my beneficial bacteria? I always put tap water in and then conditioner in after I fill the water up.

 

[DragonScales+Doubletails]

yes, some bacteria will die from chlorine exposure. but it's really nothing major to worry about. unless of course you have an ammonia spike leading to fish deaths, then there's a problem

 

[Fishmanic]

You may have some die back of the BB but if you added enough dechlorinator soon after filling and mixed it into the water thoroughly, the BB should come back. If no fish are in the tank, you will need an ammonia source to feed the BB and cycle the tank.

You are always better off adding the dechlorinator to the water before putting it into the tank or if you’re doing a partial water change, you could add dechlorinator to the tank and then add water.

 

[Slaphppy7]

I add dechlor as I'm filling my tanks during WC's

 

[Byron]

Agree with most others here. It is the fish not the bacteria that are most in danger from chlorine during water changes. You are dealing with a new tank with no fish present, so adding dechlorinator during or after the refill is not going to kill the nitrifying bacteria in the filter or the substrate. But if there were fish in the tank, they can be rapidly affected.