Light fixture sizing?

[elephantnose]

when buying a light is the size given the actually size of the plastic shell. Or the size light strip bulb that would fit?

 

[Old Scales]

Usually it's the size of the whole plastic hood. For example, my 30 inch Aquaticlife fixture is 30" but uses 24" bulbs inside it. A 10 gal. hood at 20" uses an 18" bulb if it's flourescent.
LED lights are different because they don't have to allow room for ballasts. OS.

 

[elephantnose]

Usually it's the size of the whole plastic hood. For example, my 30 inch Aquaticlife fixture is 30" but uses 24" bulbs inside it. A 10 gal. hood at 20" uses an 18" bulb if it's flourescent.
LED lights are different because they don't have to allow room for ballasts. OS.

Thanks. I was trying to find a fixture that would fit this lid. But it's a weird length.c