What is the most effective way to introduce oxygen into my reservoir tank?

 The most effective way to introduce Oxygen into your tank is by dissolving it at the surface. The surface level of the water is called the meniscus and it has been demonstrated that about 90% of the dissolved oxygen, known as DO, in a system is introduced at this point. Contrary to popular belief air-stones and bubble devices attached to air pumps are not very effective at all, in fact the only way in which they really add oxygen to the tank is by causing disruption of the surface layer allowing dissolution of free oxygen from the atmosphere. The bubbles themselves are too small and too inhibited by surface tension to release significant amounts of oxygen into the solution. You can get far more agitation of the surface layer, and a much greater Oxygen gain, by using your existing pumping capacity. The main requirement is a decent sized pump that has at least twice the required capacity. Excess pump capacity can then be used in a bypass, as shown in the following image. This bypass layout has many advantages. It allows you to turn off tap B, shutting off supply to the plants. All the pump capacity will now be directed through the bypass causing considerable agitation in the tank. Now is the time to add your nutrient concentrate and to check and correct pH. These jobs will be quick and easy as there is no nutrient in the system and the mixing is quickly performed inside the tank. Once the solution is correct the tap can be opened to send it out to the plants once more. It is usual to keep the tap to the plants wide open (B) and then close down the bypass tap (A) until flow rate through the system is correct. This should leave plenty of flow through the bypass and churning tank. If this does not happen I would suggest a bigger pump. To increase the agitation of the tank surface the return pipe should be situated well above the level so that it falls back in. This may be somewhat noisy but it will certainly improve the dissolution of oxygen at the surface level. Note that the bypass should be situated just above the pump. This will ensure that the pump is drawing water with the maximum possible Oxygen load. Once there is adequate movement on the tank surface it is important to consider the air movement in the area. If your tank is sealed, or has a lid, there will be inadequate fresh air at the interface and no oxygen gain. Cannabis roots give off various compounds during the growing cycle, most commonly gases of the ethylene family which are heavier than air and which can flow down the gullies and collect in the tank. If the tank is sealed then the agitated nutrient could be dissolving ethylene rather than oxygen. This could possibly contribute to the kind of anaerobic environment favoured by Pythium and other pathogens. It is very important to keep your tank uncovered, or only partially covered and make sure that there is adequate provision for air movement across the surface. A small fan blowing across the top of your tank, or through an opening in the lid will be a very good idea indeed. Fresh air blowing across a highly agitated surface will be far more useful in terms of Oxygen than air stones could ever be!