to me, looks like some type of nutrient deficency....are you fertlizing on a regular basis? do you have diy or pressurized co2? what lights are you running?
+1.your light is weak, i use to have the same light on my 37 (similar dimensions to yours). My plants were slowly dying from the bottom up. Upgraded to t5no and they bounced back. I also started to dose dry ferts and it helped greatly.