Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Well, it depends on what you have the room for.
Most of the fish you buy are usually small enough that a month or so quarantined in a 10 is just fine. Having a 20 long will be good for those larger fish that are still juveniles.
10 gal and 20 long are close enough to the same height that it will not make a lot of difference in plant height when they are used as grow out tanks.
Having at least one 10 on hand could be useful down the road if you need to hospitalize any fish. Many meds are dosed based on 10 gallon volume of water, and it is more economical to dose 10 gallons than 20. Of course a larger fish would be better hospitalized in a larger tank.
OK, I think I am coming down on the side of the mix of 10s and 20s. There are uses for each.
Get the quarantine tanks set up and cycling so they are ready to handle the fish when you start buying.
You can also do a fishless cycle in a bucket of water to have more cycled media ready whenever you need it. I like the Aquaclear filters for this concept: The media is cycled by being loose in the bucket. When I want to set up another tank I just stuff some of that media into a filter with a large chamber and the tank is ready to go, at least for the few fish that I buy at any one time. Add new media to the bucket and it grows bacteria quickly with the other (cycled) media in there to act as a seed source. Sort of like a revolving cycle.