Lions are great tank fish. The most venomous of fish is the Reef Stonefish (Synanceia verrucosa), and the most poisonous fish is one of the puffers, most probably Takifugu rubripes
And damn are stonefish tough...I once had mine in a big rubbermade tub on the basement floor, and when doing WCs I forgot to turn his power bar back on...lived for a week without heater, filter or oxygenation before I noticed lol. Should sell 'em in bowls instead of bettas
PS don't buy one unless you have mucho experience with venomous fish like scorpionfish first...They are potentially deadly...