My plants are drying out – the leaves at the top of the plants start curling and getting brown spots before the edges completely die and all that’s left is a pitiful inner area around the main leaf stem. I’m using an ebb and flow hydroponic system with a half strength dose of General Hydroponics fertilizer, I clean out the water and equipment every month and haven’t noticed any excessive white build up on any of the surfaces or tubing. My closet garden is about a meter square with a meter and a half of vertical grow space (approximately 3.5 feet square with 5 feet vertical space). The plants aren’t growing more than 60 cm (2 feet) tall before they start to have problems so I’ve never had to raise my 400-watt metal halide lamp. I pump the hot air out of the garden every hour with a large fan. I can’t seem to find any bugs or sign of disease other than the tops of the plants just wilting and dying.
You didn’t mention how much space is between the tops of your plants and the lamp, or any type of air circulation system but, from what you’ve told me, I believe your plants are simply overheating and getting burnt by the lamp. Try placing your hand over the top of the plants for 30 seconds – if it is uncomfortable then the lamp is too close. You can raise the lamp to a better height for now while you set up a simple circulation system to cool down the garden. Another idea is to exchange the air in the garden more often, or set up a system that changes the air constantly; some growers use small fans and hoses to remove the heat of the lamp directly so that it never has a chance to burn the plants. This method works best in small areas as it allows a few plants to grow very close to the light.