peace lily needs help!

[cin]

I have a peace lily that has developed large brown areas on large portions of most leaves. I've read this could be over or underwatering or salt buildup? However, I do not know from which my poor plant is suffering. This is not just brown tips but large dead areas anywhere on the leaf and it is affecting most if not all of the leaves. Any help would be appreciated! I'm also wondering what to do with the leaves already affected by the brown spots - leave them be, trim them back?

I've attached 2 photos so you can get an idea of what I mean

THANKS

[Rima]

How's the humidity there? And does it get lots of water? Mine were growing in soil, I washed it off about 1/3 of the plant (+ roots) and kept it in my 10 gal. tank with my betta (Siam. fighting fish). I kept having to cut back the roots, which exploded, and now, tho' the in-soil part of the plant is gone for various reasons, the fishtank part - now out of the tank but still in water only, is growing and flowering still!