Aside from testing parameters regularly? Because I was also told it's a bad idea to use old water as well.
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So this is all just my opinion so I hope no one gets to crazy about it lol! In my experience you can use some of your old water maybe 50-75 gallons of it but majority you want to use is new. replenish all the trace minerals and such!
Ok. I keep getting told that my fish corals and inverts will die due to the cycle if I do this. If I set it up that way to avoid an ammonia spike is there anything that I need to do?
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Now for this, is there a chance your fish and inverts will die if you transfer all at once YES, does it mean they all will definitely die NO! let me explain, there are WAY to many factors for any of us to give you a definitive answer. If the live rock has a lot of organisms in it and there is a ton of die off in the transfer could hurt your parameters? that's a chance you take in doing it in one day!
Now the safest way if you have the space/time to do it is fill the new tank with new water and some of your rock let it cycle like any new tank then slowly move your livestock over. If you don't have the space or the time to do so maybe see if your LFS can hold your livestock in QT while you cycle your new tank!
Congrats on the new tank and hope this helps!