If it was only one ant it was probably a scout, keep your eyes open for more.
Yes, they can be dangerous to a t or so I've heard.
An idea I've used in the past was double sided tape.Put it around the outside of the tank, if the tape is sticky enough it should catch the ants.
Never tried on a T tank but works well in window sills.
I would definatly check for more ants. I woke up to find hundreds of them in my tank. From my own experience, I would recommend spreading about a half an inch wide layer of Vaseline on the sides of the tank so they can't climb in, and then maybe put some terro outside of the tank to kill them if they come back.
from reading your from Australia....I would be concerned about ants. Like someone else had mentioned.....its probably a scout. If you are worried about it then wipe down anywhere the ant may have left a scent trail. I heard you Ausies have some nasty ants. What kind of ant was it......do you know?
Staples is correct if you don't want to loose some spiders I would spread some Vaseline if I were you. I came home from work one day to find 6 tarantulas dead and many more under invasion. The only way I was able to stop the invasion was to spread Vaseline around every single entrance. I lost some loved ones . Tarantulas are completely defenceless against these monsters. Good luck!