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I have a couple of tanks, niether one has ever been lit. The 15 gallon has been set up for 6 months, and the 20 gallon has been set up for 3 months. Eash contains a few fish who have never been exposed to an overhead light before.

I got a light when I first got the 15 gallon, but I never used it until I rediscovered it in my closet. I've set it up over the 20 gallon (the box says it is suitable for both 15 and 20 gallons) and it makes the tank look great.

However, whenever the light is on, the fish panic, looking of rcover and swimming eratically. When I turn it off, their behavior goes back to normal.

My questions are: A) Has anyone else ever experienced this before? B) Should I just take it down, or should I attempt to aclimate the fish to the light? C) If I should try aclimating the fish, what's the best way to get them used to the light over them? Would feeding whenever I turn the light on encourage them...should I leave it on for for an alotted amount of time...and if so, how long?

Thanks for your help,

-SF

In nature if fish were exposed to bright light it would mean they can be seen by birds, and other fishes. Assuming you have reasonably small fish in your 15g, they are afraid of the light because they are afraid of being eaten.
Leave your light on for 8-10 hours on a timer. This will simulate natural daylight but may also stimulate nocturnal activity.

It would be best to leave the light on for twelve hours a day. Fish, like most animals, generally need regular photoperiodicity that is best achieved by having lights on the tank. Additionally, if you ever plan on growing plants in your tank (always recommended), aquarium lights are necessary.

Regarding the panicking when the lights are on, the fish will soon grow out of this. Just leave the lights on during the day, and they will eventually adjust. It would be good, however, to add some darker hiding places to the tank to help the fish feel more secure, if there aren't any already.

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I'm not your neighbor, you Bakersfield trash.

Start by turning the lights on for a short period of time during midday while the ambient light is at its greatest, and increase the length of time its left on gradually. They'll get used to it, just don't shock them too much. To aid in this, offer them plenty of cover, hiding places and refuge from the bright light. Some floating plants will help diffuse the light and make it easier on them. And yes, to answer your question, feeding them when the light is on will condition them to think its not such a bad thing. I've a 5g with a very bright compact floutecsent light in it, and the fish really don't appreciate it. The lid is sealed with the light inside or I'd replace it with something less intense, so I put some floating lilypads in there and it really helped.

Thanks Guys,

I plan on going to my LFS and getting some more artifical plants for a hopefully a new tank and I could use those at the surface to disrupt the light.

Also, the tank I am working with has access to window light (which has aided in the growth of your basic brown algae, my lovely combatant lately). I take it this filtered window light is not enough for the fish to remain healthy, correct?