Leaves:
Discolourations could indicate problems with nutrients, watering, diseases or physical damage.

For Example:
Bottom leaves that yellow from the centre vein out to the edges, leaving a firm bright yellow leaf that is easily removed, indicates under-watering. Leaves that yettow from the outside front of the teaf inward, feel clammy and are hard to remove, indicate over- watering. A brown, crispy, tip indicates flood watering and a soft brown tip indicates a plant that is sitting in stagnant water. If either of the brown tips show a bright yellow line between the brown of the tip and the green of the leaf, you have a high buildup of salts. Check the underside of the leaves and the nodes (the place where the leaf stem joins the main stem). On leaves near the bottom of the plant, look for webs, bumps, cotton-like fuzz or any other regular pattern of damage that could indicate insects or fungus.

Plant Shape:
Does your plant's growth pattern appear tight and close, with pale foliage? Too much light causes this symptom. A plant that is stretched and spindly means too little light. Locate your plant accordingly.

Salt Buildup:
Check the surface of the soil. And the holes in the plant container, for white grey crusty material that indicates high salts resulting from improper watering and/or overfeeding. Salts in the soil slow or reverse osmosis in the root system (not cool). In other words, the uptake of water and nuitrients by the by the root system is slowed or reversed by the buildup of salts in the soil which act like a magnet, attracting water from the plant. This buildup of salts causes the following symptom: New growth is smaller or nonexistent and any damage to foliage is bordered by a bright yellow line between the damage and the green, especially at the tips. A quick solution is to scrape off any salts around drainage holes and make sure the holes are clear, by posting a pencil (or something to pierce) into them. Remove the top quarter inch of soil since this is where the majority of salts reside. Be on the look out for and remove any osmocote slow release fertilizer which appears as small round soft beads throughout the soil ball.

Now we are ready to wash the soil. To remove the remaining salts - using rain, distilled or deionized water, place the plant pot in the bathtub or a large sink on an old towel, slowly fill the tub or whatever with tepid water (not too hot, not too cold). Continue to fill until the pot is an inch under water, then let soil ball soak for a half hour, drain and repeat. This will give a good flush. Same can be done with your hydroponic reservoir. Drain and fill with just pH water, making sure the roots are covered, let stand and repeat. After this process you will have a hungry plant, so mix a weak solution of nutrients and feed. This should be repeated every four to six months, or any time your plant appears to be failing or dying or just not healthy looking.