Most people have a ton of stuff plugged in around their tv.
We have a tv, vcr, dvd player, video rewinder, and a language filter.
I dont see how that is any different than a tank.
Extension cords, or any cord for that matter, are dangerous because they are typically exposed to more abuse. I raise dogs, and that is one thing I always tell customers who buy my puppies, he is a baby. Puppies love to chew on things, and soft cords are a favorite, Once the wire is exposed, it could produce a fire hazzard.
Also, be sure to install smoke alarms and keep the batteries fresh.
You might consider removing carpeting and installing tile around the tank, which might slow a fire if one were to start?
Have a fire extinguisher accesible.
This is good advise for anyone, rather they use lots of cords or not.