converting 55 to 90g

is it even reccomended to transfer a few gallons of water from the 55 gallon tank because its fully mature or it wouldent even matter?

 

You want to keep as much of the old tank water as you can to ease the transition of existing livestock to the new tank. It won't, however, do anything for cycling, etc. Make sure you move over as much biological filtration to the new tank from the existing one as you can.

 

+1 on above

If I where upgrading from a 55 to a 90
I would use all the water I could remove that remained clear
I would leave the last couple of cm and depending on age of sand bed
may decide to discard that also

I would use the last couple of cm of water to rinse some of my rockwork, prior to tansfering to new tank

I would use base rock to complete aquascape in the new tank, placing my mature rock on top of this (aesthetics for me as the new rock will all be very light and as such, I prefer this to be 1 level, with the mature and thus darker rockwork above this)

Steve