Can you post a picture of the plants affected? It sounds like Ca problems, but pictures and a GH test would help confirm.
Calcium nitrate (CaNO3), calcium sulfate (CaSO4) or calcium carbonate (CaCO3) will all raise the calcium levels.
Calcium nitrate (CaNO3) - will also add nitrates so don't add or reduce the nitrates added if you use this for Ca.
Calcium sulfate (CaSO4) - is a good one to add since S is not usually a problem if overdosed a little.
Calcium carbonate (CaCO3) - will raise the KH (carbonic hardness) which might raise the pH depending on your CO2. CaCO3 will also dissolve very slowly, so it might be good if you want a long term solution, or if you have soft water. It can be added with crushed coral, shells, or in chemical form.
Site where you can buy these chemicals. http://greenleafaquariums.com/aquarium-fertilizer.html