This is something I wrote on another site:
THE 8 STEP RECUPERATION AND RECOVERY REMEDY
1. Check the roots. If they are discolored, reddish or brown, or present an unpleasant odor, you have a problem. If they are weak, soft or mushy, you have a problem. Also while checking the roots, observe the temperature of the water. If it is warmer than "luke warm" you have a problem. This problem is probably what is referred to as "root rot" or a disease known as PYTHIUM. Remove the dead brown roots by trimming them away with sharp scissors. Do not leave them in the tank.
2. Check the humidity and temperature of the grow area below the lights in the "growing zone" when the lights are on. A temperature of above 82 degrees or below 67 degrees will slow growth, but it is not a serious problem that will kill your plants. Temperatures below 62 degrees or above 90 degrees will stop growth. An extremely high temperature in the upper 90s or below 58 degrees can slowly result in death of your plants. The most efficient temps for growth are between 72 to 80 degrees. Any Humidity between 40 and 60 percent is acceptable and desirable.
3. Check the "lights off, nighttime" temperature. Most desirable is ten to 15 degrees cooler than the daytime "lights on" temperature, averaging 66 to 70 degrees.
4. Check the distance between the tips of the plant and the tip of the light bulb. If you observe yellowing or leaf curling tips, then move the lights one inch further away. A good rule of thumb if you use HID lights, is hold the soft palm of your hand at the leaf tip and see if the bulb is too warm to your hand. If you use Stealth Hydro's compact fluorescent bulbs, we recommend a distance of three or four inches for the 65 and 85 watt bulbs and 4 inches to five inches for the 105 watt bulbs. More mature plants can handle the bulbs slightly closer.
5. Check the position of your fans. Air movement is very necessary for the health of your plants, but too strong of a fan can cause wind burn. Direct your fan toward the tops of the plants and toward the lights. Never position the fan blowing strongly downward on the leaves.
6. Add 1/4 teaspoon of hydrogen peroxide to a quart of water and add it to the tank of six gallons already in the tank. Wait ten minutes and then turn the water and nutrition solution pump off to prepare to drain the tank. Poor at least a cup of clean water through each grow cup, onto each rockwool cube and through the hydroton rocks.
7. Drain or pump the tank empty as possible without damaging the pump by running it dry. Add two gallons of additional clean water with 1/4 teaspoon of hydrogen peroxide again and then drain it away too. Again, empty the tank as empty as possible without burning up your pump.
8. Add fresh PH balanced water and nutritional packets as prescribed. PH test it again.
. You'll get rid of root problems if you invest in 35% or 50% H202, and use inorganic nutrients.