Churning of the surface is far more effective than bubbles no matter how small they are. That's why large aquariums use powerheads, not bubblers. This is physics, so it's not an opinion or feeling.
Don't overpay for whatever nonsense that is.. just buy a small salt-water powerhead. 15 gallons is a small tank. I use a single beefy (think 20 watts?) powerhead for a 55-gallon reservoir, for reference. Get the magnet attached options, not the suction cup.
Also, over-saturating water with O2 doesn't last long. By the time it soaks in, it's probably back to equilibrium with the atmosphere.