It depends on the fish. Bottom dwelling fishes like catfish don't normally get exposed to high levels of light and this includes ultra violet (u/v) light from sunlight. Other fishes like rainbowfish and archerfish are regularly found in the top half of the water column and spend all day in the sun. Although they can also be found in shaded pools with lots of plants when it gets really hot.
Most fish (not albinos) have melanin in their skin and this helps protect them from u/v light and makes their skin darker. Albino fish do not have this. If fish are exposed to u/v light they produce more melanin to protect them from the u/v rays.
When sunlight comes in contact with skin it produces vitamin D and that is necessary for all fish and animals to develop a healthy body and immune system. Sunlight is beneficial to baby fish and all baby animals, and a certain amount is required by most adult animals to remain healthy.
Depending on your set up, it could be pure coincidence that the fish getting more sunlight have grown faster.
It could be they are all males. Male fish are generally slightly bigger than females and grow faster than females.
It could be tank size/ water volume. Fish grow faster in a larger volume of water with a bigger surface area.
Food might be a factor, however my guess is you are feeding them the same food at the same time, so probably not a factor.
Temperature is a possibility. If one tank is shaded it could be a few degrees cooler than the tank receiving sunlight, which might warm the water up a bit more. Fish grow faster in warm water.
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Sunlight has a Kelvin rating of about 5500-6500K and consists of various wavelengths of light ranging from ultra violet (u/v), blue, green, red, yellow and a number of shades in between. Blue and red light is used by plants to grow. U/V light helps increase pigment (melanin) in the skin and can help with vitamin D deficiency. Yellow and white light make it easy for us to see.
Tanning beds (at tanning salons) have ultra violet (u/v) lights in them and they are being phased out due to the number of people developing skin cancer.
Metal Halide lights produce u/v light and these were and still are frequently used over large marine aquariums to encourage coral growth. When corals are exposed to u/v light they produce a natural sunscreen that protects them from the u/v rays, fish don't produce this sunscreen. Corals also luminesce under u/v and blue light. U/V and blue light are different types of light and have different wavelengths. Blue is safe, u/v can cause damage to cells.
Fluorescent lights that produce u/v light can be bought from lighting stores or hardwares and are usually sold as "Black Lights".
Some of the newer LED light units have u/v bulbs in them. These are presumably used for marine/ coral tanks.
So yes you can buy globes that resemble sunlight but ultra violet lights are not recommended for fish tanks because prolonged exposure to u/v light can damage eyes and cause skin cancer in fish and animals (including people sitting next to the aquarium). In addition to that, bottom dwelling fishes and fishes that naturally occur in shaded areas do not tolerate high levels of u/v light and should not be exposed to something they do not naturally encounter.
In the wild, catfish and other bottom dwelling fishes might be in water that is 6 foot deep or more, and the amount of u/v light penetrating this water is going to be less than the amount of u/v light penetrating an aquarium that is 18 inches high and has 16 inches of water in.
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People do find that fish kept outdoors where they get some sunlight each day, are generally brighter and more colourful than their indoor counterparts and this is primarily due to the sunlight. If you do want to have u/v lights on the tank, have normal lights on too and add a small amount of u/v light and gradually build up the time the u/v light is on.
eg: have normal lights on for a few hours and then turn the u/v light on for a couple of minutes. Then turn the u/v off until the next day. Have a couple of minutes of u/v light each day for a week and then increase it by a couple of minutes each week. Work up to no more than 60 minutes a day and 30 minutes a day is probably sufficient.
If you have u/v lights on the tank for an hour a day then most naturally coloured mid to surface dwelling fish should be ok. However, I would not have a u/v light on any tank containing catfish, eels, loaches or albino fishes.
