I would not put a Clown in anything less than a 20g.
With a 10-20g, I would not run a skimmer. If you did frequent enough water changes, you should be able to keep parameters in check that way.
For a small tank, I would probably just buy from the LFS
if it was only $1 a gallon. Less work, IMO
. If you decide to mix your own, I would use at least RO
. Tap can be questionable, as there could be things in it that you can't test for.
Small tanks can fluctuate quickly, so I would not say that they are "hard", you just have to keep a closer eye on them.