Yellow leaves

w tank
>I got my new 75 planted 3 weeks ago and all appeared fine 
>until today. Anyway I use tetra tablets in the substrate,
>and Flourish fertilizers. The water parameters are as follows
>gh=7 dgh
>kh=4 dkh
>I use 2 175 watt halides, 250 watt cable heat, yeast Co2,
>and a trickle filter. Temp is at 80 degrees.
>Anyway my contortion (?) val has turned greyish and will
>probably be removed and some of my other plants have yelow

I would guess either magnesium or nitrogen deficiency (though I'm not sure).
I've had the same problem with fungus.  At first I tried removing the fungus
but it just grew back.  I finally took the driftwood out, stuck it in a pot,
and boiled it for an hour.  Rinsed in water and stuck it back in.  No
problems afterward.

Hope this helps,