Then the coupling between the pump side and the Arduino side could be inductive. Separating the 24V wiring as far as possible from the 5/12V wiring is first step. Twisting the 5/12V wiring into a bundle, likewise twisting the 24V into its own bundle, and routing the bundles so that they are at right angles to each other for as far as possible might help.
If the coupling is capacitive, then shielding either of the two bundles will prevent that. So will separating the bundles as suggested above.
One other possibility: Even though the two sets of supplies (5/12V and 24V) are isolated on their output side, they ultimately share a common power feed? Any chance that there is a common-mode impedance between the inputs to the respective supplies and the power company like a filter or skinny line cord?