I water DAILY--that's your problem--Your plants are waterlogged and struggling to survive. Keep it up and they will die. You should only water plants when they are dry, and that means letting them dry at LEAST half way down the basket. A plant will recover in a few hours from a wilt from lack of water and simply collapse and die from overwatering.

A pot may look dry on top and still have plenty of moisture below. If you are unsure stick your finger in the soil. If it feels wet don't water. You should water late in the day--leave enough time for the plants to dry before nightfall or early in the morning. Try to water the soil not the plants.

Petunias may wilt in the hot sun of midday but perk up as it cools. That's the nature of the plant not an indication that it needs water. If it's still wilted in cooler evening, then water.

Petunias don't need excess fertilizer either. If you are using water soluble fertilizer then fertilize every 3 weeks.