Going to a 125gal tank should be fine because with the added water it will be the equivalent of a 28% water change. Going to a 150gal tank would be the equivalent of doing a 40% water change which depending on what you have in the 90gal would probably be too much. You also need to consider if you're using the exact same equipment for the new tank or if you'll be using new equipment. If you start with brand new filters you'll need time to cycle but if you bring over the current filters then they will be fine (unless they're sized for less than the 125gal tank). But if you're moving from a 90gal you might want to increase the new one as much as possilbe and go with the 150gal, if not bigger. Which means you will diffinitly want to take the time to cycle a new tank with new equipment.
