Tetra Question

Question
I've had many freshwater fish tanks over the years.  I've become somewhat knowledgable about it.  Fought an awful case of ick once and saved all the fish.  YAY!  Anyway, I recently bought my little sister a fish tank and I saw the oddest behavior.  In this particular tank, I've had a horrible problem with algae.  I've had to change the water very frequently.  Eventually I went for help and found out having the tank in direct light wasn't such a good idea.  I closed the blinds to the surrounding windows, cleaned the water again, replaced the filter and turned off the light for several days.  My sister feeds the fish but I've been checking periodically on the water and all has been well.  I finally beat the problem.  About a week after cleaning it out, I went to take a closer look at the fish.  They looked horrible!  All her tetras were sickly and pale.  Some were even begin to show the signs of leaning to their side.  They had no energy and refused to eat.  Some wouldn't leave the bottom.  There was very little movement from any of them.  I was sure they had caught something and we were going to lose them all.  ...I don't know what inspired me to leave the light on but it was the only thing I had changed in the past week (that didn't have to do with improving the water), so leave it on I did, thinking maybe they needed it and depriving them had somehow hurt them.  I checked on them about two hours later and what a change!  Their color was back.  They were full of energy.  I even put a little more food in the tank and they eagerly ate.  I've never seen such a quick turn around.  The tank and the fish are fine; I'm just kind of curious as to what happened.  I don't know if it was the light or something else.  I've tried researching on the web about it but I haven't found a similar case.  Have you ever heard of anything like this and, if so, do you have an explanation?

Answer
Hello Amanda,
As far as I can tell it simply has to do with a fish's natural biorhythm. They need a night and day cycle just like we do, and for a fish that is used to bright sunny waters during the day, all of a sudden having no light whatsoever can be just as big a shock as say a sudden temperature change.
There is also the possibility that their bodies were switched onto "hibernation" mode and their metabolism and slowed way down, making them lose their appetite and eventually become or at least seem sickly and pale.
Make sure their water is warm (75-85 degrees F) and their metabolism's should be back up to par.