There’s no single number that can tell you how long to leave the lights on in your fish tank, because it depends on a few different factors. Take a look at your situation, both in the tank and out, and you’ll be able to answer the lighting question in no time.
Your fish need to have some light every day, though just how much they need can vary quite a bit by species. Fish that normally live in muddy, murky water don’t need a lot of light, while fish that call sparkling clear waters home need a lot. The young of light-loving fish such as guppies may develop twisted spines or other deformities if they don’t get enough light as they grow. If you aren’t sure exactly what your fish need, a good goal is about four to eight hours a day, which should be satisfactory for most tanks.
An aquarium with live plants in it has more specific lighting requirements, and unless you have strong lighting in the room where your tank is, the illumination will have to come from the aquarium light. Your plants will need between 10 and 14 hours a day of light, so that’s how long your light needs to be on. Fluorescent lights are fine for most types of aquarium plants, plus they have the advantage of being both inexpensive to run and cool, so they don’t hear up the water.
Take a look at how much other light is around. An aquarium in a room with uncovered windows or a light that is on a lot doesn’t need as much as a tank that sits in a dark room does. Your fish don’t necessarily need their tank really bright, and in fact too much light may even bother them, but you should be able to look in and easily see all parts of the tank. If not, the lighting needs a boost and you should turn on the light in the tank. If you can easily see into the tank, you may just want to use the aquarium light for special occasions, because your fish don’t really need it.
Keeping your aquarium brightly lit encourages the growth of algae. While many types of fish eat algae as a part of their regular diets, in most cases there are not enough fish in a tank to keep up with the rapid growth of this pesky plant. If you have the light on a lot and you notice the sides of the tank always seem to be covered with green slime, cut back on the number of hours the light is on each day. Try cutting back by about half an hour and see if that helps. If not, cut off another half-hour, and keep backing up until you find the right balance.