They are getting too much or too little of something. Check the suggested living conditions for them, amount and temp of water, room temp, amount of sun light etc.
Plants take in their nutrition from soil. The nutrients in soil and water we pour is sucked in from the root and travel up the plant. the edges of leaves may be drying due to lack of water content in the soil that provides nutrition and facilitates the travel of nutrients to the entire plant. so water the plants more.
If the edges go from green to brown, the soil is probably staying too moist and is essentially suffocating the plant. Some plants are susceptible to Flourine damage which is in tapwater and causes the tips or edges to turn brown. If the edges turn yellow before turning brown it could be from underwatering or a nitrogen deficiency.
There are many plant problems that have this symptom. Including: Root Rot from over watering, drought stress, salt damage from over fertilizing or particular nutrient deficiencies or toxicities.
