Coffee contains about five times more fluoride than drinking water and tea has even far more than that. Plant foods are sold as salts or as salts dissolved in water. Salts are stable compounds that, when dissolved in water, can be absorbed by plants. Fertilizer solutions that aren't taken up by a plant often dry out and so revert to their salty forms. Excess fertilizing can cause brown leaf tips.
Unused salts build up in over fertilized soil and damage a plant's roots.
This malady can occur even if you fertilize only now and then. If you use too much fertilizer at one time, it won't all be absorbed. Worse, a sick or dormant plant isn't able to absorb even, a normal dose of plant food. So feeding a sick plant can harm it. Root hairs that are surrounded by excess salts are injured and, therefore, aren't able to send water up to the plant.