I know that the tank water contains very little Beneficial bacteria, but that’s the only thing that comes to mind. Could the leaves be holding some sort of bacteria on them? Bacteria can form in many different places.
Anyways if you have fish there its probably a good thing that the tank is...
Is it affecting any other fish? If it doesn’t
it could be some sort of genetic disorder.
I would just keep watching to see if any of the other fish are affected. And than see if it is infectious, and than formulate the next step.
If it is harnessed tightly it should work. But I believe a tank lid would be better.
With a mesh, if a cat stood on it, the harness might give way and let the cat fall in. The reptile mesh was to keep animals in not out, but than again it depends on how tightly it is harnessed.
As said before it is best to prize the health of the established fish, before that of the new arrivals. The guppies may have been previously diseased. I’d set up another tank just to QT them and make sure the rest of your stick doesn’t get infected.