light hurts fish

Question
Dear Chris,
 Hello, it's me again. After all my fish died last time I took a long time before getting more. I belive the problem was that I misjudged the size of the tank and over medicated them. I haven't put anything in the water except some liquid microbacteria, and just a bit of that. I didn't put anything in the water to declorinate it, I just let it sit for 5 days with the filter running.  The water in Taiwan is very bad.
 Now the problem. Yesterday when I got the fish they were all fine. I got two bottom feeders (yellow with black tiger-stripes) and some food for them. Today, I found one laying on the bottom of the tank and the other hiding under something.  I turned off the light to see what would happen.  The hider came out and started scutting around.  The other perked up but didn't mover around much.  When I turned the light on again, the hider hid again and the other flopped on his side.  I've got the light off now, but the little fish (guppies?) are laying on the bottom when it's dark.
 I've had trouble with fish and light before. I was keeping two newts in this tank with a small light just on one side. After a few days, the gravel and the decorations (little fence) had turned a burnt orange color. It cleaned off easily. I'm worried that there is something in the water that is reacting with the light.  Or could it be the microbacteria that I added in?
 Also, the one sick (non-hiding) bottom feeder's head looks black unlike the hider.
 I don't know about the ph, nitrate, or ammonia values or how to go about measuring them.
 Hope you can help, it would be quite depressing to loose any more of these guys.  Thanks,
Bill

Answer
Hi Bill;

So sorry you lost all your fish. Very sad indeed.

The fish you have now are acting pretty normal actually. Bottom feeders are typically nocturnal creatures. When the light goes on they go crazy for a bit. They find a place to hide and then stay there until the lights go out again. Many of them lay around and "play dead" too. I wouldn't worry unless they show physical signs of disease like tattered fins, cloudy eyes, spots, etc. Color variations are normal in each fish too. Some have darker body parts than others. This can change with their mood, surroundings, time of day, temperature, even the weather.

The guppies are on the bottom because they "sleep" at night.
They aren't nocturnal and can't see so they rest on the bottom. Normal too.

The burnt orange color is simply algae. Wipe it off and make a water change. This way some of it will be removed from the tank with the dirty water so it can't fall back down and start growing again. This kind of algae is most common in newer tanks, tanks with lower light, or tanks that aren't maintained on a regular basis.

Remember about "New Tank Syndrome" too. Let me know if you need information about it.

Followups welcome.

At Your Service;
Chris Robbins