Planted Tank Guru
Join Date: Jul 2008
Location: East Lansing, MI, USA
I have both T5HO and T5NO fixtures.
I keep only T5HO lamps on hand. (As the ballasts die, I'm installing LEDs in the fixtures)
Electronic ballasts effectively read the electrical characteristics of the lamp and adjust to that lamp. Most, but not all, electronic ballasts can handle a variety of lamps and lamp types. Some, much better than others.
In my experience, and based on observation - not measurements - T5HO lamps in a T5NO fixture run a bit brighter than the T5NO they were intended for, but less bright than in a T5HO fixture.
T5NO will run in a T5HO fixture, and a bit brighter than in a T5NO fixture. They are slightly over driven and lamp life is shorter, but you should change them before they burn out anyway.
If you need or wish to stock up on 27w or 54w T5HO lamps, check out your local hydroponics shop. They sell more daylight lamps in a month than a fish store will sell in several years. Prices can be very good.