ok, first response. from Bulk reef supply. not really definitive, but its something.
I asked about PC's and T-5's forgot to ask them about MH's
Justin,
Different people have different opinions on this one. Some people really go all out and change them every 6 months, others let them go until they die. Personally on my own tank as well as the tank at the office, we change them out every year and that is what I would recommend for both.
Brandon...
Customer Care Manager
Bulk Reef Supply