What is the difference between T5 and T8 lighting? And what do the different coloured bulbs do?

The ‘T’ stands for tubular and the number is the diameter of the bulb. T5’s running costs are 3-5 times more because they are a higher output than T8 bulbs. The T5 is a good choice for more advanced planted freshwater and reef saltwater aquariums. T8’s are the more common bulb used in basic aquariums. They are cheaper to run and can still be used in reef tanks but won’t be very effective for coral growth especially hard corals.