Hydroponics is one of those subjects that few know much if anything about, is rife with misinformation and a minefield of useless products.

Water temperature - Your nutrient solution should feel COLD at 21oC NOT WARM. 21 degrees centigrade is the ideal temperature, it's warm enough to stimulate good growth while cold enough to be able to carry the maximum amount of oxygen (O2) for our purposes. As the water rises above this temperature the water can carry less and less oxygen.

Timers on NFT systems - Once the roots are out of your 3" cubes or baskets, you should run your nutrient pump 24 hours a day, certainly never have it off for more than 15 minutes at a time, even during lights off. Otherwise brown coloured cell damage may occur on the root surface as the cooling caused by evaporation of moisture off the roots damage it's surface. It's often better to run the solution all the time, this also helps maintain constant root zone temperature.

CO2 and Carbon in your nutrient solution - The vegetative leafy part of the plant above the surface feeds on Carbon dioxide. The roots feed on and in association with Oxygen. There is no benefit to putting carbon or carbon dioxide into the nutrient solution on it's own. However some of the additives do contain some carbon based compounds in association with other ingredients.

Nutrient meter reading - cF properties of water vary from locations to locations, often dramatically. As such it is impractical to simply state an optimum reading to aim for. cF meters are more useful for maintaining the correct strength on a day to day basis in re-circulating hydroponics systems once the optimum has already been determined. The meter can then be use to fine tune an make small adjustments to the intensity in order to find the optimal strength.

Feed strength - Not all plants feed the same amount, not even close relatives of the same varieties so don't believe anyone who tells you otherwise! Environmental conditions also affect feeding, more light = more food. As a general guide use 1/4 to 1/2 strength hydroponics nutrient solution for young plants, and 1/2 to 3/4 strength for larger plants. Most manufacturers full strength is equivalent to about 1200ppm or cF17 (with water cF 0), this is good for most species and varieties of plants. Some tomatoes may need a bit more, and lettuces would require a chunk less but you realy need to experiment with the specific variety of plant you are using to determine the optimum feed strength.

Leaf colour loss and associated rusty spots - These two things, particularly when associated together are usually caused by some environmental factor and NOT a nutrient deficiency. Unless you are seriously underfeeding or overfeeding, the feed strength you use will generally not be the cause. It is more likely to be the light too close, nutrient too warm, not enough air exchange, too cold nights, wind burn, grow room too dry or another environmental factor. Don't always believe the worst and assume that your feed strength is wrong.

Stretching - Plants can stretch or elongate. They may do this if there is not enough light, they may also do this if they have too much light or heat because the light is too close. Or it may be just genetics. There is no single reason that can be pinpointed as the cause of your plants stretching - don't believe anyone who tells you there is!

Input Air - As a hobby grower, for the best results, ideally use indoor air as input air for your grow room and expel to the outside. Outdoor fresh air inputted into the grow room can cause temperature, humidity and bug problems. Indoor air from a house has a better temperature and higher CO2 content, the extracted air should then ideally leave the building or be extracted to another room or area. Don't assume that input air from outside will be "cleaner" or better for your plants - it's simply not the case!

Air Cooled Shades - In line fans attached cool shades/ Jetstreams/ air cooled lights must blow through the shade. Sucking air through the shade will NOT work as well at cooling your lamp.

pH Adjustment in re-circulating systems - in re-circulating hydroponics systems including gro-tanks and particularly when using systems with rockwool slabs and hard water, the pH will often rise on a daily basis. Don't adjust it right down every day or you may end up with nutrient lock out from excess phosphoric acid. Instead don't be afraid to let it drift up for a day or two, this may be the lesser of the two evils. The plants are fragile and as such will be damaged if you attempt to adjust them. Disturb them as little as possible!

Cold - Too cold a nutrient solution or air temperature at night (lights Off) can seriously affect your yield. Your yield will benefit by keeping the temperature at stable 21 C.