Which scientific design has limitations due to oversimplification?

In a study of the effect of lawn fertilizer on the health of a backyard pond, biologists grow too colonies of duckweed in a large flask to represent pond ecosystems.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.