Regal 80... T5 and T8 Fluorescent Lighting Tubes
Aqua One have, over time, revised the lighting set up supplied with many larger aquariums from standard 1" diameter T8 fluorescent to higher output T5 tubes. These are not interchangeable so its important to make sure you order the right ones.
Have a quick look at the tubes you have - if they are 1" in diameter they are standard T8 tubes while, if they are around half of this, they will be T5. If all else fails measure the distance between the end contact pins on the tubes - T8 are 8mm apart and T5 are 5mm apart.
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