Yeah, like LICK said a paying tenant is the butter on your bread. But if you need to "lower your cholesterol" (so to speak)...
Tell them you will be increasing the rent at the end of their lease. (Keep your message as positive as possible to minimize any retaliatory brhavior.) Say your expenses are up and you have no choice but to raise their rent. You know this may make things hard for them, so you will allow them to move out before the lease ends, with 30 days notice.
Increased rent you tell them should be somewhere just above the highest rents in your area, as long as your property condition warrants that price spike.