Here on our farm we used three types of aeration. The first is natural wind. This is by far the cheapest and easiest. Where we live in the northeast of Brazil, it is windy a good portion of the year. This always causes ripples in our lakes. Any time the surface of the water is moved or disturbed, air is getting into the water. This is suitable if you don't have many fish and a constant wind. In many places the wind is decreased at night. This is what happened here. We would wake to find our fish gasping for air on the surface. Trust me when I tell you, this is a depressing sight. The lack of air in the water is also increased if there is algae in your lake as this consumes oxygen overnight, thus depleting the oxygen available for your fish.
The second method we used was floating aeration systems. Depending on the depth of your lake, there are various models you can buy. Some need to be in at least 1.5 meters of water. It sucks water up from the bottom and throws it out the top. It is like a big fountain. We left ours on 24 hours a day, although I know some who only turn it on at night. It is worth bearing in mind the cost of your electricity. We benefit from a rural electricity rate and the cost was low to run these machines. We secured ours by running lines from one site of the lake to the other.
Another type of floating aerator is one with paddles. This can be used in shallower water, and this breaks the surface of the water with paddles. Like a paddleboat, as it digs down into the water air is being forced in.
Another type we used in our tanks, was a blower which was connected to several bubblers. Ours was actually a commercial vacuum which sucks on one side and blows on the other. We also used a compressor. These were connected through flexible hose to bubblers or disk aerators which we suspended about two feet into our tanks. These can also be used in a lake, however the pressure to push the air down to a depth you require may be too great. It is also worth mentioning here, the size of the bubbles produced is critical as that will determine the amount of oxygen being delivered in the water.
One problem we had was how to sink the hose and to keep it from kinking on its way to the disk aerators. We decided to use small lead weights which are used for diving, these we attached with plastic coated wire. Some times the solution to a problem isn't costly, it just needs to work.