tips of my oldest leaves are turning brown

just like it says the tips of my oldest leaves are turning brown and idk why. im using a 20 watt salt lamp and a heat lamp. the heat lamp is about a foot or more away from the plants and the salt lamp is about 3 to 4 inches depending on the plant. they get watered every 1-3 days and they get 15 hours of light and hrs without

wots a salt lamp?

tips are where the leaves release the most toxins, it's the end of the road for the stems. if the leaves are a nice green and not curling ur fine.