These are the most common problems people encounter growing hydroponically:

Yellowing bottom leaves/older growth

The Nitrogen story:

Nitrogen is a transferable element (this means the plant can move it around as needed). If a plant is not receiving enough Nitrogen from the roots then it will rob Nitrogen from the older growth. In Hydroponics, usually the pH is too high and has locked out the available Nitrogen. Always check the pH before increasing nutrient level.

Save the plant: Leach! Check the pH, and adjust if necessary to 5.8 - 6.3. Check and maintain nutrient level. You may foliar feed (spray) with a pinch of CaNO3 (Calcium Nitrate) in a Litre of pH balanced water for quick results.

Leaf tips curl up

This is usually a Magnesium deficiency caused by a too low pH level.

Save the plant: Leach, check and adjust the pH level. You also may foliar feed (spray) with a pinch of MgSO4 (Magnesium Sulphate) in a Litre of pH balanced water for quick results.

Leaf tips curl under / leaf tip burn (browning)

The Nutrient level is too high.

Save the plant: Leach and decrease the nutrient level.

Leaching

Leaching should be done at every reservoir change and before countering any problem. This will rid the medium and root zone of toxic salt build up.

To leach, rinse the root zone with straight pH balanced water.

Use twice as much liquid as the hydroponic container would have held when empty.

Homegrown Hydroponics Inc. - http://www.hydroponics.com