| | Re: Disease or Deficiency in Amazon Sword Plant
I'm not familiar with amazon swords or how sensitive they are to certain things, but just from reading your information, I'd say it might be a phosphate deficiency, but I'm not too sure if it'd be logical that the amazon sword would be showing it first (it could be).
Phosphate deficiency typically result in stunted growth and a darker then green color, which fits your symptoms pretty closely.
I'd recommend getting some Flourish Phosphorus. That's a liquid addition. If you can't find that, you can order a pound of K2PO4 and mix your own solution. A pound of the stuff will last you quite awhile.