This is a modified list from the one that can be found in the Krib at their Plant defficiancy page.

Problems can be a little difficult to nail down at times. Most of the time the problem you see in a plant will either be a symptom that can lead to multiple deficiencies or a symptom that looks like a deficiency but really may be caused by something else. Water changes are strongly recommended on an every other week basis especially if you add a daily fertilizer. One of the larger reasons for that is to limit the build up of any one element. For example copper it an essential trace element but in excess it will severely harm plants. One of the ways it is harmful is in that a excess of any one element can inhibit the uptake of other nutrients. So a plant may show a symptom of nitrogen deficiency but your nitrate test shows 40ppm nitrate to high. The plant way very well be limited in nitrogen but because magnesium is in such excess it blocks the proper use of available nitrogen. Another problem commonly seen is an improper pH when the pH is off the scale the plants will have difficulty absorbing the correct amount of nutrients. Another limiting factor may be improper light or inadequate supplement of nutrients. Keep in mind that a plant tank needs balance and anything but balance will lead to a problem tank but a well balanced tank is quite forgiving.
One other thing that needs mentioning are chemicals the nasty kind. Snail irradiation remedies are copper based products and while stressing a lot of types of fish they can also lead to plant stress. Algaecides are chemicals designed to kill plants and will cause harm to a planted tank. There is also a whole array of fish ailment medications that can and will stress the plants to some degree.
So when hunting for a answer to why you particular plant is not quite performing the way you would like consider more then just a possible lack of any given nutrient but maybe the addition of some other chemical or environmental parameter that could prevent a plant from obtaining all it needs.
One closing thought salt while beneficial for some fish can be harmful to some plants and while 12 hours of light is great for the tank 18 is not always better. Algae may be annoying but is also an indication of a nutrient problem/balance in the tank but that is another paper.

What leaves
Leaves turn yellowish (*)

Small stunted plants with very large root systems; leaves smaller and lighter in color than normal; slow growth. Paleness will start at the tips of the lower leaves. If this deficiency continues, the foliage will continue to develop, but stems will be spindly, sappy and soft, flowering will be delayed, small fruit will grow and the plant will be more susceptible to disease.

Premature leaf fall-off Similar to nitrogen deficiency.

Stunted plants with dark, dull, and sometimes discolored leaves, unusually hard stems, poor root system and very little branching. Attacks lower, more mature leaves first. Occurs especially when nitrogen level is low.

Damage and die off of growing points Yellowish leaf edges.

Underdeveloped roots are the first to be affected. Younger leaves will be immobile and their edges will curl. Plants will be stunted and have dark, crankily leaves.

Yellow spots (*)

Symptoms do not appear until the deficiency is well established. The plant will be stunted. Leaf veins will stay green while the remainder of the leaf turns yellow. Brow spots will appear and the plant will dry out. Flowers will be slow to develop, if at all. Flowers that do grow will be lackluster.

Yellow areas, then withering of leaf edges and tips

In early stages, yellowing and curling of older leaves. Newer leaves will begin to droop. Older leaves then become blotchy and scorched. Flowers are lackluster, and stems are soft. The plants will be more susceptible to diseases.

Similar to nitrogen deficiency
Leaves turn yellow and become brittle and glassy an disintegrate Greenish nerves enclosing yellow leaf tissue First seen in fast growing plants.

Tips of new leaves will become either pale of yellow, and this will spread inward. The leaf will likely turn blotchy from a lack of green pigment, eventually turning brown and drying out.

Dead yellowish tissue between leaf nerves where the veins remain green.

Poor blooming, weak growth. Leaves may turn yellow or blotchy.

Dead leaf tips and withered edges
Cause - Unbalanced fertilization with iron
Remedy - Use balanced fertilizer
Yellowish areas between nerves, Starting at leaf tip and edges.

Growth will be stunted.

Dead shoot tips, new side shoots also die.

Brittle stems, and immobile new leaves with brown tips.

Yellow spots between leaf nerves, then brownish areas along edges. Inhibited flowering

(*) The plants may also become reddish from the presence of the red pigment anthocyanin. (**) Although Jacobsen does not differentiate between new and old leaves, David Whittacker reports from a hydoponics book that boron, calcium, copper, iron, manganese and sulfur are immobile elements and whose deficiencies affect new leaves. -------------------------------------------------------------------------