I want to downsize from a 125g reef to a 55g frag tank at the end of the summer and was wondering if I can just use water from my 125 to fill the 55. like 30g from the 125 and 20 being new saltwater.
that would be just fine, i think the 30 and 20 is a good idea, but if you do get new sand you may see a spike, i might be wrong, but i would just monitor for the first couple of weeks.
well i would say there shouldnt be a problem. as long as the rocks dont sit out in the open with no water or anything so there would be die off, i dont think there would be any kind of spike, your basically doing a big water change.