Not enough information to reply to this problem.
We need to know lighting - what kind and many watts?
I assume this is a 20 gallon tank?
an amazon sword will totally fill a 20gallon tank in under a year...it's a tankbuster plant)
The short answer is: you have a nutrient imbalance...probably the phosphates are too high..but it could also be too high a level of nitrates. We'll need test results for both of those as well as what I mentioned above.