I mix coconut husk charcoal into my potting soil mix. The charcoal will help to regulate the pH of the water I use to feed my plants, and is a very effective remedy for acidic water. Acidic water will prevent a plant from absorbing macro and micronutrients, causing the plant to form deficiencies and become sick.
Most brown spots, curled leaves, mottling, and yellowing can be attributed to deficiencies. A combination of these symptoms is often a sign that the water is bad. If the plant cannot absorb nutrients, then you will see symptoms of multiple deficiencies. You can buy pH neutralizers to add to your water, but the coconut charcoal lasts much longer and is much cheaper. You can also use wood ash from almost any other species except for acidic wood like ash and cherry.