Good afternoon. I have questions on transitioning from a stocked 10g aquarium to a 20g (tall) one. I plan on using the oversize filter that is on my 10g on the new tank. Would I be able to add 5gallons of water to the new tank and cycle between the two tanks adding a gallon of water every other day until I had enough water to fill the 20g, and then transfer the fish? Assuming both tanks were heated appropriately and had filters running, I plan on reusing the gravel in the new tank as well. I can put the original 10g filter back on one of the tanks. Seems like it should work, rather than replacing water in the existing tank, I’m just adding water and capacity. I appreciate the feedback.
I would get the fishies into some Ziploc bags, put them in a bucket so they don't see all the colors of the world and get stressed, transfer the water and gravel to the new tank, fill the rest with treated water and them have them acclimate normally to the new tank. I've done it this way a couple times with my tanks. Even when setting up new tanks I usually start with 5-10 gallons of established/"dirty" water to take care of the nitrogen cycle. (What you plan is just fine, I'm just not that patient and do it all at once)
Just transfer your decor and substrate and about 5g of watet and treat it as a large water change and you will be fine.
The majority of the beneficial bacteria is not in the water column, it's in the substrate and filter media. Do this: Set up your new tank the way you want it. Add temperature matched water. Shift the old filter onto the new tank. Plan to run it in parallel with the new filter for two weeks. Or forever. More filtration is always good. Move your fish to their new home. Break down the old tank. Clean up. If you are considering moving the old substrate to the new tank, then you have to bucket the fish first. Other than that, it's much the same process, although generally speaking, it pays to have more towels and dishpans around to keep your workspace neat.
