On a square grid, it is probably going to be difficult to simulate that the diagonal moves are longer than the horizontal and vertical moves, assuming that ants always move one square per tick (or none). Since the diagonal distance is longer, the ants would effectively have to "run faster" than for the horizontal/vertical moves. This is probably not what you want.
Instead of a square grid, you may therefore want to consider a grid or network of nodes all with equal distance, i.e. a hexagonal grid. This will also change the number of neighboring cells but that is the whole point.
Regarding diffusion: This is a matter of getting the parameters right. Sounds like diffusion per tick was too high. Also it should be in the right proportion relative to the pheromone production by ants. Note that the type of grid also affects diffusion.