The cross-sectional area of the water going over the weir changed when the gutter guard was added. Area is just length x height and both changed because the mesh has supports that block the area of water in both directions. So if you want height to NOT change then you need to change the length - you need to build a longer/wider guard for the overflow. Not sure what the shape of your overflow box is but you need some sort of guard that is offset an inch (or more...) from the rim of the box. Will take some trial and error.
This! ^
The gutter guard has reduced the area through/over which water can flow, so the water level rises to compensate. It isn't even a stable arrangement; as the holes in the gutter guard become gradually clogged with particulate matter, algae, etc. you will find the water level in the tank continues to slowly rise to overcome this obstruction. If any of the older members remember the old DLS (double layer spiral) medium used in corner overflows many years ago, they recall how the water level in the overflow rose gradually as the medium clogged; that's exactly what is happening here, except it's the water level in the tank itself that is being controlled and determined by the degree of clogging/obstruction.
I find that these types of overflows are pretty useless when keeping very small fish. They're even worse when you factor in live plants, which always produce a bunch of small particulates and pieces that clog overflows quickly. Then, when you remove the obstruction by cleaning, the water level in the tank drops immediately back down to its starting point, and the sump water level rises dramatically, perhaps to the overflowing point.
IMHO, the best kind of overflow is a simple hole drilled in the side of the tank near the top, equipped with a bulkhead. Inside the tank, a T is fitted to the bulkhead; one side of the T is equipped with a piece of pipe that extends down into the tank and draws water from any level you choose. I often swiss-cheese that pipe, cap the end and then loosly wrap it with coarse mechanical filtration medium; the coarseness and the large area of the medium cause clogging to be very, very slow.
The other side of the T faces the surface and is equipped with a 90-degree elbow. This is just a slip-fit, not glued in, and can be rotated so that the water surface is just lapping at the opening of the elbow. If something happens to obstruct the pipe below, causing the water level in the tank to rise, then as soon as it begins to overflow the elbow it drains safely into the sump as planned. This can also be set so that a tiny amount of the regular water volume being fed to the sump is skimmed from the surface, if desired.
The system works...it allows stable control of water level exactly where you want it...it can be combined with a similar T fitting on the outside of the tank if desired for even greater range of control...but it doesn't look cool.
