A healthy eel will most likely never have or spread a protazoan disease, so I wouldn't be concerned with ick or velvet. But I would be concerned with internal parasites, as eels have a very high incidence in this regard. So keeping them in an observation tank will make it easier if you have to treat for internal parasites, and gives you the opportunity to get them accustomed to living in captivity; eating dead food and establishing a feeding routine. Sometimes a newly acquired eel may need live food to get them settled in, and establishing a feeding routine with dead food may initially present a challenge. Your display will be safer to introduce a moray after they have been accustomed to eating dead food and is on a regular feeding schedule.