Poisonous: reflects something that is inherently poison...a more broad band delivery system of skin, flesh, fluids, etc. The "meat" is poison.
Venomous: requires a specific delivery system...spur, stinger, fang, spine, etc. That provides a mechanical point of contact for venom/poison.
Toxic: this expresses a value of caustic affect by means of a dramatic change that causes a measure of unhealthly material...shock, stress, illness, death are common "release" mechanisums for the toxic quality found in nature.
With this in mind, the most poisonous fish is an interesting one. Because the rating of a given animals poison(venom) qualities are based upon the strength, speed of affect, duration, and if there is a anti-venom. Most poisonous fish, as noted above is dealing with the "meat" or fluids...and venomous deals with the commonly used reference of poison creatures.
So I am assuming that you are looking for the the most venomous fish? If you are still looking for the most poison fish, I would say blowfish (fugu) or red sea puffer with tetrodoxin, is pretty high on that list. Most venomous, is most likely going to be a pacific stone fish.
Good luck with your project.
Ray Boemler
www.peanutbutterjellyfish.com