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Do you plan on upgrading the filter as well or just use the old filter?
Do you plan to move the gravel over too?
If you simply move over the old filter and gravel to your new tank (without letting them dry on the way, and do it fast), you get to keep almost 100% of the beneficial bacteria. Because these good bacteria are mostly colonizing on your filter media, the remaining are mostly in the gravel but not nearly as much as in the filter media.
The old tank water holds next to 0 beneficial bacteria because they only colonize surfaces. So there is no value in transferring the old water for nitrogen cycle. However, by transferring old water, you actually avoid the chance to shock the fish in new water. even though you use the same water source, the water PH and hardness in the tank as well as the nitrate concentration can be different in the tank and fresh tap water. So you do get some benefits by using the old water.
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