Not any harder, just have to keep your salinity adjusted correctly, be sure to pre-mix/dilute all salt that goes in the tank while fish are in it. I haven't tried puffers as i hear they are fin-nippers, since my eels are mainly fin i don't think that would go well- lol.
Look into archer fish (youtube), they are next on my list- seem like a pretty neat fish. or violet gobies, mono's, scats. The biggest fish my eel has eaten was 2 6 inch needlenose. They are Predators always.