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Most problems in a grow look the same from a distance. The leaves discolour, growth slows, something is clearly wrong, and the instinct is to feed the plant. Feeding is the wrong first move often enough to be worth a guide of its own.
Start with the root zone, not the leaf
Three things cause most symptoms, and none of them is a missing nutrient.
Water. Too much of it is the classic. Roots need oxygen as much as they need moisture, and saturated medium holds none, so the plant droops in a way that looks like thirst and gets worse every time you water it. The giveaway is the weight: a heavy pot under a drooping plant is waterlogged, a light one is dry. Let the medium dry back between waterings and the droop lifts within a day.
pH. A plant can only take nutrients up across a narrow band, roughly 6.0 to 7.0 in soil and 5.5 to 6.5 in coco. Outside that band the feed is there but unavailable, and the plant shows a textbook deficiency while sitting in plenty. That is why deficiency symptoms so often survive extra feeding: the concentration was never the problem.
Too much feed. Burnt, curling leaf tips, browning from the edge inwards. Nutrient lines are sold at the top of their range, and most schedules run happily at half to three-quarters strength.
Reading the leaves
Where a symptom appears tells you more than what colour it is.
Older, lower leaves yellowing evenly while the plant otherwise grows: usually nitrogen, and normal late in flowering when the plant is pulling reserves out of them.
New growth affected first, pale or twisted: more likely an immobile nutrient, calcium, magnesium or iron, and in coco very often calcium and magnesium, which soil supplies but coco does not.
Bleached, near-white tops on the tallest plant: light, not feed. The lamp is too close.
Spots, edges and irregular patterns rather than an even fade: check for pests underneath the leaf before adjusting anything.
Pests, briefly
Spider mites leave fine speckling and, once established, webbing; they thrive in warm dry air. Fungus gnats live in wet topsoil and are a symptom of overwatering as much as a pest in their own right. Thrips leave silvery trails. All three are far easier to prevent than to remove: airflow, a lid on the humidity, and not letting the medium sit wet.
Mould, which is different
Grey mould, botrytis, starts inside dense flowers where moisture cannot escape, and it is the one problem that is not recoverable. Affected material goes in the bin, not into a jar, and the surrounding flowers get checked daily. Prevention is airflow and humidity control in late flowering, and outdoors it is a matter of variety choice and timing, which the outdoor guide covers.
A working order of checks
- Lift the pot. Wet or dry?
- Measure the pH of what goes in and, if you can, what runs out.
- Look under the leaves.
- Check the distance to the lamp.
- Only then consider the feed, and consider reducing it before increasing it.
Most of what a grow needs is stability rather than intervention: a temperature that does not swing, a medium that dries and is watered again, and a schedule that stays put. The indoor basics guide covers the conditions those checks assume.