To your question: I have a pair of percs in a 29G along with 1 CBS, 1 Pep Shrimp, 1 turbo snail. I do not run a skimmer on this tank. Its reef with corals, etc, though I do run a cannister filter with carbon and rowaphos. I perform 1 5G wc
once per month. no3
are undectable so the bio load is well withing the 29G limit IMO
Some Advice: It seems you have added 1 clown already. Its really best to add a pair that are not 'mated' at the same time to prevent territory issues. However, that doesn't mean it cannot be done. I would strongly suggest you find a clown that is either distinctively larger or smaller than the one you have now. The female will 'become' larger and more aggressive. When you do add the 2nd (I wouldn't wait any longer than need be) re-arrange some of the aquascape to help ease any potential territory issues. The clown is in the damselfish genre and can get quite aggressive, so try to make it as easy as possible. Consider also that even with all these steps, they may never be a 'mated' pair per se. A little nipping and fighting even up to 6 months or so is not uncommon, but a female that bites and causes injury to the other is a sure sign they need to be separated. Not trying to talk you out of it, matter fact its a great idea. Just be aware the sooner you introduce the new 'mate' the less chance of territory based aggression.