Is this a bad idea?

[Aquattro]

I might use the water from the existing sump (it holds about 60 gallons) but the sand is new, rock is new but partially cured/seeded, some existing rock will move over eventually.
Fish load is medium for 180, 15 fish total. 5 butterflies, a tang, a wrasse, 4 clowns and 3 damsels. Pretty sure I'll need to have new water on hand in case of a spike.

[PoonTang]

Is it possible to move half of your fish, wait a bit to make sure that everything is ok, then move the other half a week or so later? Or move all or as much of your existing rock over as possible, even if it is only to the sump for a bit. Then you will know for sure that your bacteria will handle your bioload.

[Aquattro]

Rock in the sump is a good idea, I'll do that.

[PoonTang]

Originally Posted by Aquattro

Rock in the sump is a good idea, I'll do that.

also FWIW everything I have read suggests that transferring over your old water will do nothing to help out the cycle but it will help acclimate the fish a little better.

[Aquattro]

Originally Posted by PoonTang

also FWIW everything I have read suggests that transferring over your old water will do nothing to help out the cycle but it will help acclimate the fish a little better.

Agreed. Bacterial content is insignificant, but chemistry could make transition for the fish easier. I prefer to start with all new water though, as I'm switching salt between tanks.

[sphelps]

Can you just move everything over at once? If you move all the water and rock from the 90 over as well you shouldn't have any cycle problems, it's really the same as just doing a large water change. I've done this a few times with large upgrades without problems.

[Aquattro]

Originally Posted by sphelps

Can you just move everything over at once? If you move all the water and rock from the 90 over as well you shouldn't have any cycle problems, it's really the same as just doing a large water change. I've done this a few times with large upgrades without problems.

The sump won't fit all the rock from the 90, but I'll put 40 or 50 pounds in. The trasfer might take a while as I'm cutting all the corals off the existing rock and remounting to the new rock. So I have to at least keep the 90 3/4 full and just use the water from the sump.

[Myka]

If not already mentioned, watch pH closely...turning the skimmer off like that can cause pH to drop quickly enough to cause trouble. If that happens just add a big airstone.