[kevmo911]

In simple terms, more surface area equals more diffusion. A certain amount of air split into larger bubbles has less total surface area than that same amount of air split into smaller bubbles. You can do the math involving surface area of a sphere if you'd like. Or you can trust me - I did do the math

In addition, the longer there's contact between air and water, the more diffusion will take place. Because of this, larger bubbles, which travel to the surface faster, will diffuse less air into the water than smaller bubbles that hang around in the tank. This is also the reason that some people (like me) place diffusers in front of a powerhead or under the outflow from a HOB to blow the bubbles around for a while, rather than allowing them to travel directly to the surface.

Because airstones don't create very small bubbles, they're not very effective in terms of diffusion. Atomic-style diffusers, which create very small bubbles, are far more effective. But they do create surface agitation, though this is more effective in smaller tanks (because of the smaller volume and surface area) or with multiple airstones.

Hope this clears some stuff up

 

[kevmo911]

If you have too much flow, just downgrade to a smaller HOB.

But it really sounds like you're looking for a reason to go with a canister. Just do it. Start cheap, with a SunSun. Look up HW-302 or 402. They're the white canisters with black and blue trim on top. The 302/402 are the lowest flow ones. For comparison, one of them is decent for my 30g.