Lighting is just as important for fish as for most other animals. This means that fish should have appropriate light, of appropriate intensity for an appropriate amount of time.
The most common method of measuring light in an aquarium is by adding up total wattage of bulbs used and dividing that by the volume of the water, in US gallons. The 1-2 wpg rule of thumb was based on the old T12 lights. Now, the thinner and more efficient T5 and T8 fluorescent tubes are used for most home aquairia. So for modern fluorescent lights in aquariums under 60 cm tall, around 1 wpg is best for the average, undemanding plants, combined with average water changes and average stocking, although it is not uncommon to find anything from 0.5 wpg, in low-plant aquariums, up to 2 wpg, in “high-tech” (meaning fertiliser and CO2 addition to encourage plant growth and prevent algae). I find that generic tri-phosphor fluorescent tubes work just as well as “aquarium” tubes, even for plants, so I will normally buy those to replace old tubes over planted aquaria and I use old tubes from planted aquaria in plant-free setups until the tubes break.
LED lights are also increasing in popularity now and compact fluorescent bulbs are a popular choice for DIY units.
There are few problems associated with low light, with the exception of poor plant growth. On the other hand, high levels of light, long photoperiods, multiple lighting periods per day and photoperiods of varying length encourage algae growth, which can become a long term problem. A timer and a photoperiod of 6-10 hours per day usually go a long way to preventing potential problems. Another point to consider is that some fish require light to be not too bright, or they will feel uncomfortable. In these cases, it is best to provide floating plants, which will cut down on the amount of light entering the water and will give the fish places to hide, thus making them feel more comfortable.
I normally use mechanical timers because they are reliable enough and cheaper than digital timers, but this time, I found a digital timer which was only 2 EUR more than a small mechanical timer with half hour intervals and half the size of a mechanical timers with quarter hour intervals, so it was an easy decision.
The light which was included in the set came is a 24 watt compact fluorescent (also known as a power compact) bulb, referred to as “Plant Pro” by Arcadia. Given that the aquarium will hold about 16 gallons, that gives me approximately 1.5 wpg. It is more than I’m used to, but I have occasionally had success with that much light without addition of fertiliser and CO2.