What is the fish stock, size of the tank, and filtration setup? Possibly underfiltered and/or overstocked.
Did you clean your filters the last water change? That could induce the cycle.
The oil slick on top is a bacterial colony usually comes from a combo of not enough surface agitation and to many nutrients in the water (overfeeding?). The milky water is most likely your tank going through a mini-cycle again since the water change.
Even without further info your best bet is to continue doing 25-50% water changes daily. This would be ideal and give the fish the best chance to survive the cycle.
If you have some seachem prime you can double the dose or go even higher (exact dose on the bottle) to help with the chemicals present from cycling.