Stem Plant decay

I have a rather heavily planted 55 gallon tank with a ph of 7.0, kh of 2
and gh of 6.  I have been using a DIY C02 injection system for about 2 
months now via an airstone.  For about the first month the plants were 
doing extremely well.  As a matter of fact, the Hygro, Foxtail, and 
watersprite were growing many inches a week.  However, for the past
2 weeks I have noticed that the Hygro, Foxtail, and Tropic Sunset
have started to decay wherever they were snipped and at the base of
the stem.  There are even cases where they were rotting in the middle
of the stem.  I removed the bands around the stems when I planted them.

Could this rotting be caused by a lack of fertilization.  I have only been
putting about 2 cap fulls of ferovite in the tank once every three weeks
because I was having a problem with thread algae.  Also, I have been 
doing 30% weekly water changes.  Is this too much?

Thanks in advance for any advice.