I think you're more likely to be injured or killed by fingernail clippers than any tropical fish in an aquarium! ... And if there's any aquarium fish that's lethal to humans, I've sure never heard of it.
There is the electric catfish also known as the electric eel which is not actually an eel but a fish.
This fish can send out enough electricity to cause heart failure or at the very least high amounts of electricity which will be very painful but luckily you won't see it in most peoples homes not only because of it's huge size but also the sheer danger of keeping them which means they should only be handled by professionals with rubber suits and rubber gloves to avoid electrification .
Some are dangerous because they have very sharp teeth such as snakeheads or goliath tigerfish that can bite off fingers very easily if they feel threatened so these are also fish that should only be kept by professionals with huge aquariums and lot's of space .
Alligator gars are also monster fish that can pose a threat to humans because of those huge teeth.
Piranhas also deserve a mention but these guys will only pose a threat to a fishkeeper if they are not fed enough food because if they starve they will go for anything that moves even if that means cannibalism or shredding the hands or fingers of an unfortunate fishkeeper and if that happens it usually means the fishkeeper is not skilled enough to keep them in the first place.
What is more likely to cause fatalities though are certain types of corals in the reef keeping hobby because when handled incorrectly for example while cleaning they can release toxic substances in the air which can turn fatal quickly if not treated immediately
When it comes to popular fish kept by common folks the piranha is the most likely to cause severe injury but only when neglected and letting it starve which might happen under the care of a new fish keeper who got a piranha because it's "cool" because they saw some movie about piranhas and how ferocious they are which is usually not the case and only under certain conditions.
Even though the pufferfish might seem like it's only dangerous when eaten unprepared the truth is that pufferfish have very sharp teeth which are used to eat hard shelled creatures like clams but that also means if they bite you they will tear pieces of flesh off.
In conclusion anything that has a large amount of teeth large enough to harm you and the agressive nature to back it up such as goliath tigerfish or anything toxic enough to kill a human are fish that should be cautioned keeping unless you are an expert with the right tank setup and the right type of gear.
