Different aquariums need different types of lighting and understanding this is one of the keys to successful aquarium keeping.
Fish and aquarium lighting
Unlike plants, fish do not carry out photosynthesis and are therefore not as depended on light for their day to day survival. There are actually quite a lot of fish species that have adapted to a life in caves or deep down in the ocean where there is no light at all. Most aquarium species will however appreciate a sense of day and night in the aquarium, since this is what they are used to from the wild. Developing a colorful breeding coloration would for instance be useless if the pitch dark environment made it impossible for any other fish to see it.
Incandescent or fluorescent lights?
Since fish do not carry out photosynthesis, they do not really care about your choice of light. Incandescent lights will however produce a lot of heat and can make the water near the lights really warm. Since the fluctuating water temperatures can be harmful to your fish, fluorescent lights are a safer choice. They are a bit more expensive, but will on the other hand cut money off your electricity bill. Fluorescent lights are also more similar to sun light and will make the fish look more colorful and brilliant in the aquarium.
How many hours?
If you have fish that hails from the tropics or subtropics, they will prefer roughly 12 hours of light a day, year round. Fish that live further away from the equator are used to long summer days and short winter days. The easiest way of providing your fish with a stable environment light wise if of course to connect the aquarium lights to a timer.
Too much light?
While most fish species appreciates a sense of day and night, strong light can actually make them shy and some will even dampen down their colors. This is especially true for species that hail from murky waters or environments where the sun is filtered through floating plants or the canopies of overhanging trees. Adding floating plants to the set up will normally solve the problem.
Too much light will also promote excessive algae growth in the aquarium. In a fish only aquarium, no more than 3 watts of lighting per gallon of water is recommended. If you keep live plants you can use stronger lights without causing an algae bloom since live plants and algae compete for the same nutrients.
Plants and aquarium lighting
How much light your planted aquarium will need depends on which plant species you chose. It is therefore important to find out the light requirements of a plant species before you decide to use it in your aquarium. There are many plant species that will thrive even if you place them in an aquarium with no other source of light than normal, fluorescent aquarium lighting intended for fish-only aquariums. In the other end to the spectrum you will find extremely light demanding plant species that need the strongest available type of fluorescent lights or even more potent light fixtures to survive and retain their color. Be especially careful with species that grow close to the surface in clear tropical waters. Plant species that develop red leaves are also notorious for being extremely light craving.
Corals and aquarium lighting
Keeping corals is popular among saltwater aquarists, but most corals will need really powerful lighting and not even the strongest fluorescent lights will be enough for them. Metal halide lights and similar are therefore common in such aquariums. There are coral species that grow far down in the deep parts of the ocean where light is scarce, but a vast majority of the popular aquarium species grows close to the surface under the strong tropical sun and forms symbiotic relationships with light demanding green algae that carries out photosynthesis
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