I have used pure H2O2 both in waste water driven (very high BOD) fish tanks and fish farm tanks during hours of power break down - no circulation. Oxygen will spread very fast when added inside the tanks. . In one case we withhold oxygen saturation down to bottom in a 3 m deep 40 000 L tank with only pure H2O2 loaded water into the first 30 cm of the tank depth for 18 hours. I confirm both concentrations and spread with an electronic probe. It was a waste water driven tank (very high BOD5) and the only movement was created by the fishes. It was not dense populated - it was an example and show tank. To use H2O2 and also oxydators at power break has been my standard method for years. Its my experiences that diffusion between two media (air and water) is slow but spread (caused by different levels of concentration inside a media) is rather fast, especially if the gas is introduced in the bottom layers. Its can also be that way that H2O2 in contact with water (and especially saltwater) will cause a rather high oversaturation in the contact area, hence a higher equalization effect due to high concentration differences
IMO - very surprising - density of O2 = 1.42 g/L - density of water 1000 g/L. I´m surprised that only molecule movement can mix surface oxygen into the bottom in only 60 hours. In our case - with oxydators - the oxygen will enter the water column in the bottom of the aquarium and add vertical movement into the molecule movement.
When we did experiment with emergency use of pure oxygen in fish tanks - we get the best result if we combine the size of the bubbles and the overpressure of the O2 in that way that the bubbles barely reached the surface or very few of them get up in the air . We did not need to have diffusers over the whole bottom area - it was enough to have a diffuser tube around the sides. We confirm both concentrations and spread with Oxygen probes
Sincerely Lasse