One of the most important factors in successfully keeping fish is providing enough oxygen. Without enough oxygen, your fish will be found either gasping for air at the surface, or floating upside down.
If you look at aquariums you will notice that they almost always use bubble stones of one variety or another, where a compressor pumps air into the stone and it comes out small pores in the stone, causing a thin stream of bubbles. These are generally fine for very small tanks, but they are really inefficient in terms of how much oxygen they add to the water for the amount of electricity used.
All methods of adding oxygen to the water rely on increasing the surface area where the water and air touch. Not only do the water molecules need to be adjacent to the air molecules, but you need to constantly replace well oxygenated water with water that needs oxygen.
One method of increasing oxygen is to have a very high surface area and low depth to your tank. With less water, you naturally have a lower stocking density (assuming you have a good fish to gallon of water ratio). With a large surface area you have more natural water/air interaction. Unfortunately you will also have a lot more evaporation, your tank will take up more space (in terms of square footage), and it may become difficult to cover the tank or reach the middle. The major advantage of this method is that a loss of electricity will never kill your fish.
There are scaled up versions of the fish tank bubblers for larger tanks of fish. There are also EPDM or Polyethylene liner based bubble disks which do much the same thing as the bubble stones. In order for any of these to really work well, it is essential that you have very strong air pumps, such that there is a lot of churn at the surface. Ideally you would have the churn rise above the surface of the water by at least two inches (2" or 5cm). A churning action increases this surface area significantly, and provides an opportunity to mix low oxygen water with the air. The column of bubbles will also move low oxygen water from the bottom of the tank towards the surface. There are two problems with this method of oxygenating the water.
- The very expensive high pressure air pump you will need for a tank with any significant depth. It will both be expensive to purchase, and will cost a lot in terms of electricity.
- There will be a whole lot of evaporation occurring where the bubbles escape the water. This will be evident if you cover your tank where the bubbles surface.
Fountains and waterfalls are both pretty and help solve the oxygen issue. Both fountains and waterfalls utilize a standard sump pump or inline pump instead of an air pump. Avoid the splashing where the wind and sun hit the tank to prevent excessive evaporation. The biggest issue you will have to contend with is keeping the pump from clogging. Using filters, such as mesh pump bags, plastic filter media, swirl filters, and settling tanks will be needed, along with regular maintenance.
Pond aerators are not often used with smaller tanks, but should be considered if your fish will be in a really large tank or pond. These are murder on the evaporation rate, and require some significant energy to operate. On the other hand they add a lot of oxygen, don't clog, and are very reliable.
My favorite method for aerating my fish tanks is to use air lifts. An air lift utilizes the action of air rising through a pipe to push water up the pipe with the air. While air lifts can be used to move water a significant height above the water line, for aeration we really only want to go an inch or so above the water. I recommend a simple and cheap low pressure, high volume air pump for this application. Use 1/2 grey (electrical) pvc pipe with a 90° pvc fitting on top. I cut the bottom at a 45° angle to let in water, and I drill a 1/4" (6.25mm) hole about half way down the pipe (less for deep tanks > 4ft or 1.2m). Into the hole I squeeze a 1/4" (outer diameter) drip line which is connected to the air pump. For larger pumps, use an Air Divider like the ones provided by Aquafarm. You can either suspend these, attach a float at the water line, or you can group several together to make them stand up. The benefits are:
- low cost and quiet air pumps
- low risk of clogging (they do occasionally clog with algae)
- easy to construct
- low evaporation (lowest in my opinion, for the amount of oxygen added)