Everyone posting is currently assuming that you have a bichir, but I'm not so sure. I'd hold off until you have a better ID on your fish, if that's a dragon goby it will need brackish water, not to mention your tank would be small for it. Can you find a picture of your fish? All of the fish mentioned are much better off in fresh water though a few can stand slightly brackish conditions.
Peacock eels are rather shy, and while your tank would be large enough to keep them I'd be very careful about tank mates, keep them peaceful. Tire track eels are more agressive and do well with cichlids and other agressive fish though they'd need a larger tank of 80-90 gallons at least. Fire eels get much too large for the size of tank you're talking about, I'd avoid them. Knife fish don't come small, and I really wouldn't even try one in a 90 gallon tank, maybe if you started talking 200-300 gallons then you'd be able to keep a knife fish and/or a fire eel.
If you're looking to keep something interesting in your current tank, there are all sorts of colorful cichlids with great personality and they come in all sizes. There are cichlids suitable for tanks as small as 10 gallons up to larger species where a single fish will need at least 75 gallons to himself. Check out the cichlid sections for more information on the different species, also check out the oddball and brackish sections for more interesting fish that will fit in your tank.

There's also much more to interesting fish than size, my little loaches have color, personality and get along great in community settings, they're my favorites even having kept spiny eels, puffers and cichlids. Bettas and gouramis can be very personable fish as well, many get along fine in smaller tanks.