What kind of plants are you talking about. I own a greenhouse and have grown commercially for 15 years and I've never come across a plant that sensitive to tobacco. In the past many insecticides contained nicotine, most are off the market because they are pretty dangerous to humans. 4-5mg of nicotine would make a non tobacco user very ill. I chew so 4mg is just a nice dose, lol. Just curious what plants you feel are getting a growth regulator effect from your hands. Overall Nicotine is a VERY toxic substance, it takes less mg of nicotine to have an acute toxic effect (again assuming a non tobacco user) than it would arsenic. Overall plants are far less sensitive than inverts (including coral), birds, fish, reptiles, mammals, amphibeons [sp].(very sensitive to nicotine).

As to your tank, yes, you absolutely should wash your hands before they go in the tank. Nicotine or worse yet a tiny amount of gasoline on your hands can screw up a tank fast.