Fuses

Look at the current rating of the ballast.

soo, that would be 110V ?? Im not seeing any other numbers here .

What does the label on your ballast say? 110V is the voltage not the amperage. The ballast should have information stating how many lamps it can power along with the current (amperage). Does your fixture have a single ballast? What does it say on the lamps (F24t5HO???)

Oh I see it. There are three different lamp types and line currents for each one.
.71A
.56A
.39A
The reason I want to know how to figure out Fuse size is because I am building a light for my saltwater aquarium and the ballast I have is recommended for T8 bulbs. I researched and found this would be fine to use with the T5 bulbs I want. So this, I think, throws off any amperage info on the label...

It is always a good idea to get in touch with an electrician.