there T5 tubes ...
you have t5 tubes and t8 tubes ones bigger than the other the unit your looking at takes T8
the bulb your looking at is T5 it wont fit
T5's are better as they give out more light per W but can be more expensive.(i think)
T8's give out less light per W.
there is not a concern if you just want to see your fish. plants will grown under both T5 and T8. but you'll need more WPG with T8 for growing plants (in a high tech tank)
step 1. measure your tank
step 2. look at bulb's and find one which will fit in you tank (remember to go slightly under so you can fit the end caps of the starter unit, i forgot todo this... had to take the bulb back....)
step 3. find the wattage of the bulb
step 4. find a starter unit to power that wattage bulb
T5 and T8 wont make much difference besides the fact it wont fit your starter unit if you miss match it
step 5. buy it