It could be because they are dying or it could be due to algae. If it looks like algae your nitAtes could be high. - what are your water stats? Here are a few possibilities:
1 – You’ve got driftwood or natural shells in the tank that are decomposing. This is a source of nitrates.
2- You’re beneficial bacteria colony has become inadequate.
3- You’re not vacuuming the substrate adequately.
Bottom line, you need to identify the source of the nitrAtes and correct it if you have a high reading. If water stats are ok it may be that they are not getting enough food - you can buy a plant food to add to the water.
If they look like rotting brown or transparent then they are dying, it is okay to trim it off because it won't grow back. Trim it as close to the base as possible.
Plants prefer a temperature range from 75 to 85 degrees. Aquarium water that is too cold or too warm will eventually lead to the death of your plants. Most plants need 10 to 12 hours of light per day to thrive. Make sure that your aquarium is properly lit. A general rule of thumb for lighting aquariums that contain live plants is 2 to 5 watts per gallon.
Make sure that you have the proper amount of gravel (substrate) in your aquarium. Some deep-rooted plants need as much as two to three inches of substrate for the roots to grow properly.Take any plants out of the little pots you buy them in and plant in the substrate or the roots cant grow properly.
Unless you want to get involved in a CO2 system always choose dark green leaved plants as these tend to need less intensive light and are more hardy.