not enough oxygen in the water so they swim up to surface for air. do you have any air stone? or using filter outlet/power head to break water surface for gas exchange? water parameter, temperature, tanks size and how many p's? what size are the ps?
To add to that, tanks can get warmer during the summer, depending on where you're keeping them and your air-conditioning situation. Increased temp means less oxygen in the water. Check temp for sure.