There will undoubtedly come a point where you need to replace the lighting in your fish tank aquarium, or certainly change the tube which is there due to it having blown or getting dull. You would be faced with an array of different replacements however one thing worth noting is that there are 2 main types of aquarium light tubes which are of different size amongst other things so are not interchangeable. The 2 different types are T5 and T8.
T-5, t-5, or T5 is more efficient than T-8, especially T-5 HO (high output). The t or T number is the diameter in 1/8 inch increments with T-5 being 5/8 inch diameter and T-8 being 1 inch in diameter. Using T-5 tubes will allow you to fit more tubes in your hood and each tube will put out more light than the equivalent length T-8.
For a heavily planted aquarium or a saltwater reef tank I would always use T-5’s and not T-8’s. If it’s not a saltwater tank and you don’t have any plants, then using T-8’s is fine, though you may need more of them to get the results you want.
Hoods that are set up to accept T-5’s will not accept T-8’s as the two do not interchange. They have different pin ends and the ballasts are different for each type.
Also T-5 HO produce almost twice the power at almost half the size of T-8’s
so smaller bulb = more space
more space = more bulbs
more bulbs = more PAR/Power
more/better PAR = healthier tank
healthier tank = what you want!
Think of PAR ( Photosynthetically Active Radiation) as a measurement of the total amount of visible light that shines on a square meter when you start a stopwatch and hit stop after one second.