Lions aren't really what I would call an aggressive fish in the sense you mean it. They are very good at eating things, but that's just out of hunger (unless your title was actually referring to an aggressive fish and not just lionfish...). Eels you can keep in there aren't that aggressive either, though some of the larger ones are definitely aggressive.
If you really want something that will make you worried about doing tank maintenance that involves putting your hands in the tank, look into Pseudochromis steenei. The most aggressive dottyback, beautiful colors, teeth jutting out of its mouth, and they are even known to attack divers. If you want to make it worse, get him a female, but don't put another species in there with it. Anything that could live in a tank with it in a 36g would likely be a victim, except it may ignore a dwarf eel, but who knows.
When I was keeping one and it finally matured into a terror, it left me with several bites even when I was constantly trying to avoid its lunges, and some of them drew blood. Nothing serious because it is a fairly small fish, but for your tank size it is pretty much what I think you are looking for.
