**INVEST IN A GOOD SALTWATER AQUARIUM BOOK** before you dive into a marine tank.
10g may be okay for a single clown, I wouldn't do a pair in a 10g- most recommend 20g as the minimum size for a pair, and with any of the above that is only for the smaller clowns (percula or ocellaris). Also note that the smaller the tank, the more difficult it will be to maintain water quality and salinity level.
Very few people do filters in a reef tank unless they are changed several times a week, because they tend to trap waste and as it decays, so does water quality and thus coral health.
Skip the bubbler and go for a good nano powerhead, like a hydor koralia.
If you add tap water to the tank with live sand, then mix in the salt, you will end up with dead sand and poor water quality from dieing bacteria and such from the sand. You will need to premix saltwater before adding it to the tank (fresh water will kill live sand, live rock, corals, marine fish, snails, etc). You also need to used conditioned tap water (RO/DI water is much better, but you could likely squeak by with conditioned tap water depending on the corals you want to try). You would not be able to have any other fish in with a clown in a 10g tank - as she matures, she will not want to share the space with any other fish.
Any corals require better lighting than the junk that comes with the tank, good lights tend to be expensive but for a 10-20g it shouldn't be too bad (depending on what type of corals you want).
In any size tank, the max is 2 clowns. Once they mature, two will pair up and will kill any others. This is the case for any species. The two must be the same species (although you could do different colorations of the "designer" clowns as long as they are both the same species (percula or ocellaris)).
A lot of this info is freely available online or in a good book. If you don't do the research first, you will be one of the countless people who rush into things, end up buying stuff twice when you find the first thing you purchased was total crap / useless / etc, and ultimately trying to sell an unsuccessful SW tank on craigslist. Make sure you can commit to the total cost of the setup, the patience of letting it cycle and mature (especially before adding sensitive corals), and the weekly maintenance / daily upkeep of the tank, before you spend the time and money to do it halfway. I'm not trying to discourage, but trying to help you do it right the first time, if you decide you do indeed want to do a SW fish tank. Having a tiny tank with several clowns is not a long term dream, unless you want to constantly replace clowns....