Triton Bulbs and Stem Rot

I have a 55 gallon aquarium which currently has 3 48" bulbs (1 Triton,
1 regular, 1 coral life).  These bulbs are now 6 months old.  How
necessary is it to replace these bulbs at 6 months?  Can I wait up to
a year?  Also, should I go ahead and replace all 3 of them with 
Tritons.  I have heard that Tritons have a higher light intensity and
will also help to eliminate algae problems.  Currently, I have a slight
thread algae problem.

Also, about 2 weeks ago I posted a message about several of my 
bunch plants rotting at the stems.  My nitrates don't register on
the Tetra kit - no traces of ammonia or nitrites  either.  The PH is 6.8
and my KH is 2.5.  The water temp is 76 degrees.  I am also using a DIY CO2 
injection system which I have been using for the past 2 months.  

The plants that are rotting are the ones that I had to trim about
once a week because they were growing so fast.  These plants include
Hygro, Foxtail, and Tropic Sunset.  Most of these plants I have 
had to completing throw away.  I had only fertilized with ferovit
one time over the last month because I had a thread algae problem
which has become better.  I did not get any response to my last posting
about this problem.  If anyone has any ideas, I would appreciate hearing
about them.  I am also wondering if I can go ahead and replace these
plants or will this problem probably occur again.

Thanks in advance for any advice on these two issues.