8am-5pm is 9h, 3 pm-10 pm is 7h. It means your plants will get less light, so you might want to consider 1pm-10pm, if the plant growth was nice with 8am-5pm. If you don't want to stress your fish, you might do this change gradually. Change to 9am-6pm for two weeks, then 10am-7pm and so on. I...