Ok so you have a 20 gal tank...how much live rock to you have in it and what are your water parameters fisrt off? Secondly, did you get your clowns at an LFS or online? Normally if you order a pair of clowns online they'll be sure to send you a male-female pair, however at most LFS' they stock large ammounts of ocelaris clowns (false percs) in the same tank. When they're in large numbers together they tend not to be aggessive but if you bought two males for example, and placed them into the confines of a smaller tank such as yours, they may become aggresive towards eachother. This tension can cause stress which may be the reason the fish are not eating. Check your water parameters though...clowns, especially tank raised ocelaris tend to be a very hardy fish and can handle quick changes in PH, salinity so on and so forth. But if something is way off...like a high NO2 or NO3 level or high ammounts of ammonia, though I doubt the ammonia level is high in your case, that can very easily stress out the fish as well. Stressed out fish are more vulnerable to catching disease. For now I would venture to say that no, they are not sick...I strongly suggest checking your water parameters and if possible find the parameters of the place where you bought the two percs from. If something is way off that may explain your problem. Also consider you may have placed two aggresive fish in a smaller tank, which in turn can be causing the stress which is causing them not to eat at the moment resulting in the low ammounts of produced waste....let us know