Leaves turning transparent

[leebc at sbs_sumibe.co.jp]

Hello guys,

Recently I am facing problems of my plants rotting.  The leaves of the
plants will start to turn transparent from the veins.  It will start near
the stem and propogate to the tip of the leaves.  This has been happening
from the past 2 months.

It started with my Elantine sp(?).  It is my foreground carpet plant.  One
big patch just melted away.  I had to removed the whole carpet and save the
good stems and replanted.  The problem just disappeared.  Then the problem
hit my Hygophila stricta and A. reineckii.  I trimmed the rotted stems of
Hygrophila and threw out the all the A. reineckii.

The problem stopped for about 1 month and reappeared again last week.  My
Lobelia cardinalis and Echindoras ozelot were the latest victims.

I have not changed anything for the past 2 months.  I have been keeping to
the fertilization with Flourish, Flourish Potassium, Flourish Excel and
Flourish Tabs based on the recommended dosage.  I used Equilibrium at water
change.  I have about 35ppm CO2, 3W per gal lighting.  The temperature is
almost constant at 30 degC (86 degF).

I am really stumped by this problem.  Anyone of you can give me some advice
of what happened.

Thank you in advance for your help.

BC (Singapore)