[Alin10123]

Agreed, most of the bacteria associated with the cycle is in the filter since that's where most of the waste is when it gets sucked up. I would rinse the new sand out, put it into the new tank. Use as much as the old water as possible, then dechlorinate the new water. Then run the old filter in conjunction with the new filter in the 90 gallon tank. I wuold assume that if you do it right that you dont kill off much bacteria and you shoudl be able to have the same # of inhabitants in the new tank. Then slowly add more and more fish into there to the new stocking level. That will give the bacteria a chance to catch up.