Specially formulated hydroponic fertilizer mixtures are required and should be tested after dilution to ensure a pH level of between five and six. This nutrient solution should be changed every two weeks and in between changes should be checked so the volume is kept level by adding more water only and not additional formula. The more plants you have the hotter your greenhouse will usually be. A temperature control unit can be set to turn fans or blowers on and off at pre-determined intervals. They can also be set up to open and close air ducts in your greenhouse or grow room. As you progress in your hydroponic garden hobby you may also want to switch from an aquarium pump or bubbler system to a more advanced irrigation and drainage system for your garden. The hydroponic systems also require less space, because the plant roots don’t have to spread and search for food and water. The small space requirement makes hydroponics perfect for limited space home gardeners. The greatest benefit to hydroponics is the ability to automate the hydroponics system with timers and remote monitoring equipment. There are two basic divisions between systems: water culture and media based. Systems may also be active or passive. Active systems use pumps and usually timers and other electronic gadgets to monitor the operation. Passive systems may also incorporate any number of gadgets. These systems try not to use pumps and may rely on the use of a wicking agent to draw nutrients to the roots. In areas where food is traditionally produced, but where fields are over crowded or stripped of their nutrients, hydroponics is a way food can be produced using a minimum of space and resources. Hydroponic growth leads to an increase in production over traditional farming in two ways. For one, hydroponic gardening just plain saves space. Spider mites will be turned off by the smell of the marigolds and will leave the garden. Biological insect control is another method of pest management through the introduction of predator insects to eliminate spider mites. Using enemy insects is a safe and poison free way of controlling spider mites.