Airy bud question

My buds are really airy. I have two plants I started from seed. They are healthy and a good strain. Purple power and K2. I'm 7 weeks into flower the plants are under two 600 watt hps. They are big with lots of small airy buds like I don't have enough light. The bulbs are only 6 months old Agrosuns. They really streached in the last couple weels also. I appreciate any advice out there. It's probably to late this time, but I don't want it to happen again.