Lighting issues

Question
We currently have a 20 gallon tank, containing three white skirt and three black skirt tetras, 2 bala sharks, a plecostomus, and 1 blue gourami.  We have a 15 watt full spectrum light for the plants and fish, but when we turn it on all of the fish excluding the gourami swim to the bottom, and stay there.  This is natural for the pleco given he is a nocturnal fish, but not so much for the others as they prefer well light tanks and seem to need them for health.  When the light is turned off, they fish instantly swim to the middle and top of the tank, and act much happier.  Is our light too strong for the size of our tank?  Or does their sensitivity to the light indicate some type of disease or illness?  We preform weekly 25 percent water changes and vacuum the gravel.  We also have a heater that maintains the temperature of 78 degrees.  The filter we use a Whisper, made by Tetra, intended for a 20-40 gallon tank. We have checked our water levels regularly and the only problem we have is hard water.  The fish don't seem to mind, but we aren't sure how to change this without using chemicals.  Could drinking water bought from the store work?

Mostly we are wondering why our fish seem to not like their light when everything else seems right and they act very healthy and happy.

Thanks for any advice,
Calla and Chris

Answer
Dear Calla,
Your fishes behavior of swimming to the bottom when the lights are turned on is normal. Fish often do that because it scares them and they may also be waiting for their eyes to adjust to the light. Usually they get up and start behaving normally after about 20 minutes or so. Turning on room lights before turning on their aquarium light can help fish gradually get adjusted to the light. It's best to turn one or more room lights on for about 30 minutes and then turn their tank-light on. It may take them to awhile to adjust but just give them some time.
I don't think you have too much light, it's just about right.

It also helps to turn their tanklight off at night but have some room lights on for a while before eventually turning them off. That will help somewhat duplicate a natural sunset and help avoid scared fish from suddenly being plunged in darkness.

Also don't worry about the water hardness. Fish are very adaptable and can thrive in a huge range of water chemistry because they have been captive-raised in fish farms for so long. And trying to adjust your water chemistry tends to do more harm than good because chemicals just only work temporarily. As long as your fish have a good water temperature and always have clean water then they will surely thrive.

I really hope this helps!
Best wishes,
Karen~