Join Date: May 2015
Location: Central Michigan
If you plan on doing water changes daily or every other day, why not just put them in the display tank to begin with and use a bottled bacteria to help establish the BB colony?
I use seachem stability with all new fresh and saltwater (fowlr) tanks I setup and have had nothing but success using as directed. Zero fish loses. I've setup (mostly upgrading nanos to slightly larger nanos) about 6 tanks in the last 3 years using Stability. Transfering fish from tank to tank. I have 3 glo-light tetras and a neon all over 3 years old and they are on their 3rd tank, soon to be 4th. A pretty decent trial of the product I think. IME, it's good stuff. It supplements the BB colony so new tanks are fish safe and it helps establish the BB colony as well.
It would, IMO, be no different than dumping them all in the 10g. Unless Cardinals are overly sensitive fish, which I don't believe they are, I see no problem in doing what I suggest and do myself.
As far as a true quarantine/ hospital tank goes, a bare empty 10 gallon is plenty large for 20 tetra sized fish and for solitary fish you'd be surprised what fits in a 10g for quarantine/hospital purposes. The problem you run into here is that if a fish or two show symptoms you should then quarantine them to treat them separately from the original group. No point in 1 fish getting 19 ill too. Hence another reason why I suggest just putting them into the display tank to start.
Do a fishless cycle and worry about it much less. Using Stability, I've cycled a 12" cube in less than 3 weeks.
Daily/every other day water changes combat a lot of issues when starting new tanks.
Almost forgot to mention, if using tap water I (and pretty much anyone else) highly suggest using prime for your dechlorinator. Pretty much a staple in this hobby. It also helps reduce the toxicity of ammonia. Another great seachem product.