I've kept Fire eels, similar species, different markings. They're very nice, semi-hardy and usually will eat anything that fits into their mouth. This means that as they reach their 3ft cap, most species in peoples tanks get eaten. Your sharks for instance, if not full grown by then may be seen as a snack.
A 90gallon should be more than enough room for one, however make sure the lid is a tight fit and has no holes from which to escape as these eels can easily shoot out the top of your tank and be found dead on your floor.
I used a bare bottom tank as it is MUCH easier to maintain. Be careful with rocks if the eels cut them selves it will cause infections and most likely death. They will feed right from your hand almost immediately ex. worms.