A plant that is too far gone will look brown, and when brown>green, then it I pruning time. IME I never used scissors to cut, I pull off the leaf so that there isn't anything left to sit there and rot, or for the plant to put extra energy into to try to keep alive. I've had my plants for a year (I believe they're melon swords, maybe, maybe not, but close enough) and I think that they're just now adjusting to submersed! I think this because before the plant had grown up very fast and the leaves were all growing above the water, so I started forcing the leaves under the water and—BAM! The new growth was growing only four inches high instead of a foot! I'm posting a thread to get people's opinions ^^