If you understand what is happening at the water surface, than it becomes easy to develop ways to achieve what you want, with or without powerheads.
The exchange of gasses at the water surface is pretty much instantaneous and needs little help from us.
What we need to understand is how to maximize this to achieve as high of a rate of gas exchange as possible. not only do we want to gain oxygen from the atmosphere, we also need to release carbon dioxide and other gasses into the atmosphere.
There are three important considerations to meet to achieve the best results;
1: increase overall surface area for gas exchange.
2: do not allow any films or other layers to form on the surface which would impede gas exchange.
3: move the lower layers of water from the bottom of the tank to the top.
1) when we disturb the surface of the water and cause ripples, that increases the overall surface area available for gas exchange. Powerheads are good for this, but the same can be achieved with airstones and the returns from the filters, although not as vigorously.
2) oil slicks or a dust layer will interfere with gas exchange so the surface needs to be agitated to prevent this. Again, any means employed above will also achieve this.
Sadly many HOB
filters that have the spill over type of return do not achieve this very well at all, nor do they disturb the surface enough for added surface area. you get a clean patch right around the filter and that's it.
3) exchange the water layers in the tank. This is probably one of the more important things as water at the lower levels of the tank will quickly become oxygen depleted if the tank has poor or no circulation top to bottom.
Basic rule of thumb in fish keeping, if you remove the water from the left, return it to the right, if you remove it from the bottom, return it to the top.
One reason I hate most HOB
filters is because they spill the water right back to the intake spout and don't really move it around.
I did a "fix" that, I made an extension for the siphon tube from pvc
and ran it along the bottom and over to the opposite side of the tank so water was being pulled from one side and returned to the other, but I'm getting sidetracked...LOL
Anyway, airstones are without doubt the easiest most reliable means of moving the lower water layer to the top, and it also causes an updraft current that helps keep the substrate clean. The only concern is with salt creep in the brackish tank.
My recommendation is try a small powerhead in the brackish tank and another airstone in the other tank at the opposite side and you should be fine.
Clean surface, good circulation and exchanging the water layers is most important, take care of those three and gas exchange will be fine.