Under tank lighting

I have a 55g tank, undergravel filter 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? Could I be damaging my root
growth? Is it an unhealthy thing to do?
I don't know all of the answers, but I know Who to ask. ;)