Any decent flow of water through the system will prevent this type of stratification; it's also pretty simple to modify the overflow so that it draws most or all of the water from the bottom of the tank rather than from the surface. If the tank is drilled at the bottom and has a vertical standpipe, a larger pipe that fits around the standpipe like a loose shroud, with holes drilled at the bottom, will draw water from the lower levels. With an overflow hole drilled at the top of the tank, which is what I use, a piece of pipe that reaches to the bottom of the tank, fitted with a 90-degree elbow at the top that slips into the drilled over flow hole, does the same thing.
In either case, a hole drilled in the shroud or downpipe at the water level will continue to draw a small amount of water from the surface, removing that filmy layer into the filter.
To me, the true beauty of the sump is the ease of maintenance; set-up properly, it allows you to quickly remove/replace/clean the initial mechanical filter layer (filter sock or other). You can literally clean it every day if you want to, and very easily, thus actually removing the particulate matter from the water before it breaks down. Compared to that, canister filters are a joke. They're trendy, and they "seem" high-tech to the casual observer...but they are much more trouble to clean. The manufacturers even use that as a selling point, crowing about how long you can go between cleanings. Congratulations! You have just purchased a device which discourages you from cleaning it for long periods, and which hides the poop and debris from sight while giving it time to break down and ad to your water pollution level. The only way to work around this is to use some sort of pre-filter which allows easy cleaning...that way, they are almost as good as sumps.