Responded to a guys tank issues on a Facebook group and suggested he do weekly 70% water changes. I was told that was way too much. I was even warned that I was going to crash my cycle. Why? The nitrifying bacteria isn’t in the water!?!
People on Facebook are tossers who like giving one sentence answers for any and all subjects. Don't waste your time on it, you just bash your head against a brick wall.
eg: I saw a person who had issues with rainbowfish and she had about 50,000 response, all 1 or 2 sentences long. None of the responses helped her at all and most had nothing to do with rainbowfish. I sent her a private message explaining what she needed to know. She thanked me because I was the only one who had helped with the problems.
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The beneficial filter bacteria lives in the filter and on solid surfaces that have oxygen rich water moving over those surfaces.
You can change 100% of the tank water and not affect the filter bacteria, as long as the new water is free of chlorine/ chloramine and has a similar pH, GH, KH and temperature to the old tank water. Even then, the bacteria are tolerant of variation in water chemistry and temperature so a few points either way won't affect the filter bacteria.
Having said this, we don't change 100% of the water if there are fish in the tank because they would be left dry on the gravel. But water changes don't remove filter bacteria or screw up a cycled tank.
I did 75%+ water changes on all my tanks, and the tanks in the shops, and places I helped at and never had a cycle crash.