I have a 2watts led bulb and I am planning to use it as a lighting in my aquarium of volume 20 gal. My question will be can I use it without harming my fishes? And in general can I use any bulb as a lighting for my aquarium? Thanks a lot.
@Derrick K has the answer, first. You want to check with the specs on your aquarium. Sometimes the hood will say right inside it, or outside it, or include a sticker. You might also be able to look up the part online.
You can use any light bulb in any light socket, but it's not a good idea. You want to stick within the parameters of your particular equipment. You have to remember - this is something that's going to be on 8 or so hours a day constantly, and it can get pretty hot and become a fire hazard if not within the parameters. It can also heat the water excessively.
Well, there are definitely multiple answers to the basic question, and reading the manual as has already been suggested is typically the best bet for any lighting fixture, definitely never go HIGHER in wattage than recommended.
However, talking LED bulbs, 2 watts is so low that I seriously doubt you would ever have an issue with the heat being generated. Unfortunately the lighting industry is not at all friendly to the end user. Wattage truly only relates to how much power is used to generate the light, it in fact has no real bearing on how many lumens you will receive regardless of wattage. Lumens is the actual amount of visible light you get. In general higher wattage does end up giving a brighter light, but ONLY when comparing lighting of the same type. IE a 20 watt incandescent is much dimmer than a 20 watt fluorescent, and both are much dimmer than would be expected by the same amount of power used for a LED type of fixture.
You can easily get real technical in all of this, but looking for specific colors in LEDS will make a difference in how things show up in your tank. There are LEDs that are great for plants, but look a little purplish in color, there are of course white, red, blue etc LED bulbs as well.
If you are looking for a light to just provide general lighting, general color etc, then a standard white LED will be fine, and should not cause any issues with your aquarium, assuming of course, that the fixture for the LED is designed to protect against moisture. If you want a light that will brighten the colors of your fish, the higher bluish range is better, the brighter the better. For mood lighting, generally people prefer a more yellowish tinge etc...
I know that I purchased a fixture from walmart that is using blue tinged leds and that they are generating a lot of algae where the beams are directed on the glass, the rest of the aquarium using white leds, has no issue, so there must be something in the blue led that is feeding the algae pretty well!
For one of my 55gal tanks, the only one using LEDs right now, it has a fixture that contains 100 white leds arranged roughly 1 inch apart along both front, and back sides of the fixture and that seems to be a really bright light... beats the look on my 75gal tank that is using VHO builbs in the 750watt range each, and it takes 2 bulbs for that... big as in HUGE waste of power but the plants love it. The VHO (very high output) system is an incandescent system, and is way out of date by today's standards... I will be gladly replacing it with LEDs shortly.