Under tank lighting

I have a 55g tank w/ lots of community fish and slowly building a nice
selection of plants (water sprite, some bunch plants, etc. - looks great!) I
had a desk lamp laying around. I put a plant light in it and had it sitting
on top of my aquarium for extra light. The lamp looked kind of ugly up there
so I decided to put it under the tank inside the stand reflecting off the
white wall that is behind the tank. I noticed that it looked like a sunrise
and was really cool. I also noticed a couple of days later that  my water
turned crystal clear and then a day or two after that one of my pearl
gouramis and a tiny  platty died.  Could that intense light shining on the
underside of my tank (glass bottom) have killed the bacteria that breaks
down the funky stuff in my tank? I know this sounds like a "fish" issue
instead of a "plant" issue, but I don't know if I could also be hurting any
root growth. I've not gotten my water tested yet as this just happened last
I don't know all of the answers, but I know Who to ask. ;)