I disagree, they are great fish. And not impossible. In a 65g you really only have the option of the helmet cow. Gets to be 6" tops and can be housed in your tank for life. Another pro to this guy is that he is less likely to release toxins then other cow and box fish. Only keep one. Most of the time if a cow releases its toxins its due to competition. Keep with peaceful fish.
P.S. cowfish cant release their toxins when dead and 9/10 times dont release toxins when on their way out. Keep lots of carbon on hand just in case.
Oh and if your running extra carbon in the tank, if it does release its toxins, depending on how much it releases, the tank might not even be affected.