Bring a sick plant here. Symptoms that do not obviously belong to a named pest or disease, photographs that need a second opinion, and the root-zone problems that spend a week impersonating a nutrient deficiency before they declare themselves.
Most of what arrives in this sub-board is not what it first looks like. Yellowing from the bottom up, clawed tips, spotting and a plant that wilts despite a wet pot all have several possible causes, and the distinguishing evidence is usually in the substrate, the runoff figures and the watering routine rather than in the leaf photograph.
Two habits make a diagnosis possible. Post the environment and the numbers with the picture: substrate, pot size, days since transplant, source water, input and runoff readings, temperature and humidity, and when the plant was last watered. And photograph in white light rather than under the grow lights, because a magenta or deep red spectrum hides exactly the colour shifts that carry the diagnosis.
Where to start
Related elsewhere on the forum: Nutrient Deficiency and Toxicity: Reading Your Leaves in Fertilizers.