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The answer is that it could be. I've killed off the bacterial cycle in the past by changing too much water and being sloppy doing so. You will probably be OK if you closely match salinity and temperature, and adding the new water slowly.I just got back from vacation and my tank, understandably, is looking very dirty. I want to to a 30%-40% water change, would that be dangerous to the fish/coral?

Your water column holds some bacteria, but the vast majority of it lives in your live rock. Unless you also change out the live rock when you do a water change, or leave the rock out of the water until it dries up, you won't hurt the cycle. There are people who do 90-100% water changes on heavily fed pico reefs, with no problems whatsoever.The answer is that it could be. I've killed off the bacterial cycle in the past by changing too much water and being sloppy doing so. You will probably be OK if you closely match salinity and temperature, and adding the new water slowly.
I’ve been running a180 for 6 years. Add another 30 gallons in the sump. Mixed coral/fish. I do a 25% WC every 2-3 weeks. I have done larger changes and as others have said as long as you keep salinity and temp similar you should be fine. Your corals will thank youI just got back from vacation and my tank, understandably, is looking very dirty. I want to to a 30%-40% water change, would that be dangerous to the fish/coral?
I just got back from vacation and my tank, understandably, is looking very dirty. I want to to a 30%-40% water change, would that be dangerous to the fish/coral?

