It sounds like an early sign of pop-eye. pop-eye is a symptom not the actual ailment, so you should take measures to find the cause. But...the treatment for nitrogen supersaturation (air in eye), is to turn the powerhead water stream to the lowest setting. If you run it, then discontinue air to the PH's. If this was indeed the problem the "bubble" should go away within a few days. If no improvement is noticed, or if the condition worsens, assume the problem is bacterial in nature, and treat accordingly. HTH