I've tried searching for this already but the answers vary. Most seem to say 10 to 20 gals for a QT but this seems quite large for me as my main tanks are 54l/14g and 28l/7g (both tropical). I was thinking about a 10l/2.5g plastic one I saw yesterday which would be good space and price wise but are there any issues with this being too small for a QT? I probably wouldn't have more than 5 fish in there at once and since they'd be going in my 54l tank they won't be big fish. I was planning on using the sponge filter in there but not having the tank set up all the time. I guess I could keep the sponge in the filter of the main tank when not in use?

The other thing is that I'm planning on redoing the 28l tank completely, so was debating putting the betta who lives in there in the QT. It could be about a month he'd be in there for. Otherwise I could put him in the 54l but would need to separate him from the shrimp who are in there already.