I have a 1500 gal tank that is 6 ft deep. i just hooked up my sweetwater s21 which will only get air out of the stones when they are only 1/2 way to the bottom. Is it critical that the air stones be near the bottom? I need to know if I have to upgrade my blower or not.
It seems the idea about the gas exchange taking place at the surface is a common misconception. Here's an outtake from a response in a different thread that dealt with aeration as well.
Any water/air interfaces will exchange gases. The exchange is governed by the partial gas pressures within the air and the water and the surface area. They strive to balance. They strive to achieve saturation, which by the way is dependent on many factors including temperature and the mixture of gases in the bubble and the water. But the exchange goes both ways. If for example the water has a higher gas pressure from carbon dioxide than the air, the carbon dioxide from the water can also pass to the bubble. Smaller bubbles are better at getting the most out of your aerator. First off they have a greater surface area by volume and therefore can achieve equilibrium with the water faster. Secondly they rise slower. giving more time as well. All this together can lead to a small bubble starting it's journey upward with a relatively high concentration of oxygen and reaching the water surface the same size but with less oxygen and more Carbon Dioxide for example.
And why do you need the check valves? If you locate the blower well above the water line they shouldn't be necessary.
I use a diaphragm pump for general aeration everywhere from FTs to rafts and biofilters and a second leather piston pump to give the 2 trout tanks extra air. These small motor driven pumps have both volume and pressure resulting in a powerful column of air that rises from 20-24" air stones attached to the top of my slotted waste tubes at the center bottom of the 330 gal ibcs. This not only aerates the water but stirs the tanks heavily AND draws the waste right to the waste tubes located right under the air stones resulting in much less manual tank cleaning. (and the fish love playing in that current)
The only reason I use check valves on a few lines is to keep the water from saturating the stone and line when the air is off for some reason (like showing off the fish) and not having to wait for the water to clear before resuming aeration. More of an impatience thing. I want to see that air column before I walk away. Ck valves have far less effect on a positive displacement pump. My low pressure vane powered pond water pump on the other hand doesn't want to hear the word "check valve" if it results in any amount of back pressure.
I'm not sure there is any substitute on the market for the low pressure turbine pumps when it comes to large systems and ponds. The only way to increase pressure on a vane pump is to go to multi stages like on a vacuum cleaner or extreme high speed (very loud) like on an automobile supercharger or turbo charger. That is why they use vacuum cleaner pumps on Jecuzzi spas to get all that pressure and volume. They are generally brush motors though and not designed for 24/7 year round use to say the least.
Hey Pete, that's quite a tank. Is it in the ground? Just curious.