How can I prevent my computer from interfering with the TV signal?
Your TV, sadly, has what is referred to as a susceptibility problem -- it's vulnerable to being confused by electromagnetic trash generated by other gizmos, like air pumps and desktop PCs.  From the nature of the interfering devices, it sounds like the electromagnetic trash is being carried from your PC and air pump to your TV via the power lines.

My recommendation is to plug your TV into a high-end surge suppressor with built-in EMI filtering circuitry (such as a Tripp Lite Isobar or Isotel) -- this will reduce the amount of trash that gets into your TV, and hopefully make it so you can watch TV with the computer on.