First of all, make sure that in the aquarium in sufficient quantity was present to water the plants. A lot of them: various echinodorus, cryptocorynes, Elodea, rogolistnik, aponogeton, etc. of the Oxygen that they, as a rule, is enough to meet the needs of fish, even in the warm season, of course, provided that the inhabitants in the aquarium are relatively few. Note, however, that if plants will grow too densely, then there may be unpleasant side effects, they should from time to time to thin out.
If the plants in the aquarium for one reason or another, there are no (or very few), then it is necessary to resort to aeration, i.e. forced injection of air in water. You will need: compressor, rubber hoses and the tip of the spray gun. His task is to create output as many bubbles as possible, to increase the area of contact of the air with water ( so that more oxygen is passed into the water). Therefore, the tip is made of a material that contains many small pores.
Should provide the ability to adjust the amount of supplied air (e.g., during the cold period the fish requires much less oxygen for the reasons mentioned earlier). So either buy a compressor the capacity of which you can change or get the metal clamps (preferably the screw), with which you can "pinch" the hose, changing its cross section.
If you want to saturate with oxygen the aquarium with sea water, then one of the porous tip is not enough (especially when a large volume of the aquarium). In such cases, as a rule, use a narrow long tube-"column" with lots of small holes.