Zebra eel is my vote. Fits your geographic requirement and I have never had any problems with them being agressive or eating fish, just the occasional emerald crab.
They don't eat fish and they aren't aggressive, but they do get quite big and will still make a big contribution to waste in a 90 gallon tank.+1 for this pick, won't bother any fish at all just your crabs
This is largely moot because the OP has decided to go with a dwarf. However, echidna eels (at least snowflakes) do not keep their grinding molars for life. Once they reach sexual maturity, their teeth change and they can become a big problem for other fish in the tank.There are eels that have molar teeth and those that have sharp toothiesIf you don't want a fish eater, but don't mind crustaceans being at risk... I'd go with something like the Zebra or from the Echidna ... Fish eaters will be (among others) generally in Gymnothorax...