Did your clowns exhibit any gasping or rapid breathing signs?
How about lethargy - hanging out at bottom of tank, or coming to the top in an attempt to get air?
Did the white patches kind of look like white flaky, mucosey skin?
These are all things that I saw when my clownfish had Brooklynella. It is a particularly virulent disease that tends to strike clowns more than other fish (although any fish is susceptible). Generally, the first sign is rapid breathing. By the time white patches start to appear, the fish is usually a goner unless you act quickly. I have battled this disease and won, but it is not easy. Freshwater dips will not help. A dip/bath in formalin is the only thing that I found effective. Note - ich remedies with formalin in them were not as effective as the formalin by itself. Our little clown was dipped virtually every night until he started pulling out of it. During this time, your fish will be so sick, it will often stop eating. Sometimes they can be coaxed with brine shrimp. Oftentimes a secondary bacterial infection will set in because the fish is so immunocompromised. With my fish, it was a nasty case of fin rot, which required a long MaracynII treatment.
If you are going to get clowns again, you will likely be dealing with Brooklynella at some point in the future. Best to educate yourself ahead of time so that you can see the early signs. It is much easier to treat if you catch it in its earliest stages. For more info, you can look up Brook, brooklynella, clownfish disease or anemone fish disease in any search engine. But beware of sites that tell you a freshwater dip is a remedy. From experience, I can tell you that this does not work. Also, my clowns had Brook all throughout their hyposalinity treatment, so I do not think hypo is effective against it. Or if it is, it doesn't kill the brook fast enough.
Hope this helps.
--Christy