hi guys i have a problem and need help

one of my three four plants, the leaves are starting to look like arotting bannana w/ brown spots everywhere and they yellowing and dying off.. only one plant in seperate pot..what do u think it is?.. im gonna test the soil and water right now, i'll be back w/ more info.. here are some pics!
the last pic is the top of the plant that looks like a bannana, (it looks different from teh others.. is it trying to flower? im a newbee help

thanks, well i just tested my soil.. N- adequate P- adequate to sufficient K- Defficient to adequate.. i use poland springs for water...i have no idea.. everyone keeps saying nute burn but test dont show that, and anyone have an answer to the top of the plant looking different?

plus ive flushed the plant a few times.. my nitrogen and potass show it being low, could low nutes cause that?

What's the ph? If it's too high or low it can lock out nutes and give the appearance of deficiencies.

its about 6.5-6.8