I saw a show years ago about a guy who found out how to communicate with microscopic creatures living in a tank (air conditioner?) in his house. I do remember one part where the creature says that it is rumored that one side of their world is warm and the other is cool, at which point the man checks the temperature of the sides of the tank. The man has a way of enlarging the creature and it turns out to be hostile once it reaches a larger size. What show is this?
This reminded me of the Simpsons Treehouse of Horror episode where Lisa does exactly the same thing. Knowing that The Simpsons is very good at documenting exactly what it's parodying, I looked up that treehouse of horror episode and, sure enough, it linked me to this:
You're right, it was The Twilight Zone, specifically Season 3, Episode 28, released March 30th, 1962.
This sounds familiar - could it be this episode of Monsters (a sci-fi/horror anthology series from 1990? Micro Minds - Astronomer Dr. Thomas Becker (Troy Donahue) is upset that his graduate student, Paula, hasn't been to class. Paula reveals that she's made contact with an alien species, but the source of the communication seems to be so nearby. This means a ship must be in orbit. Dr. Becker dismisses her claims, but is convinced of the truth after he speaks to "Gok" the alien. He begins to realize he might become world famous. Paula becomes suspicious of Gok's motives, and soon discovers just what Gok really is. But can she stop Dr. Becker in time?