After water, Run and check water levels, leaks,flow in case you have to change something and once thats good, shut off pumps and then put in sand, best if its wet (should rinse it out good if its new unles its live sand from a bag, you know, store bought ) Then wait a while for cloud to settle and then turn on pumps.You will still get some clouding but it will clear up. Are you useing stuff from your other tanks for the 125g? If so then it should already be LIVE and there should not be much ammonia ect. If its all new and you have to do the typical new tank cycle then go ahead and do it the same way also. Good Luck and Congrats On the 125.