Upgrade advice

[TheFragile]

I currently have a 6.5 gallon tank running a whisper 10i filter. Getting a 20 gallon set up. I'm planning on running both filters to establish bacteria in the new filter. What is a good amount of time to do this for? How long before it would be safe to remove the old filter?

[coralbandit]

2 weeks should work IMO
You are going to move old filter and fish to new tank with it's equipment?
If so then just test after a couple days to make sure no mini cycles.
Once new filter is established then you can slowly increase stock allowing the filter to grow along with the increase.

[TheFragile]

I'm replacing the substrate as well. But everything else is going into the new tank including the fish.

[coralbandit]

You don't need to move much water from old tank if your source water is the same and you have been good with waterchanges.
Just the filter and fish need to moved to bring active healthy BB to a new tank.
I have done this many times.
Good luck.
Set up the new completely and let it get to proper temp before moving fish and filter.

[candymancan]

despite what PPL claim on this website, the vast majority of bacteria in a aquarium is actually in the substrate.. I will defend that until the day I die

[TheFragile]

I had to change it. Since I was obviously going to need more anyway, but it was neon colored for my glo tetras and for my son, and my fiancé did not want them going in the new tank that will most likely grace my living room one day