Clearly the “cycled” tank is going to become an uncycled tank as the bacteria that were present to process nitrogen wastes all died with the antibiotic treatment. The rock and sand are functionally dry rock, although we don’t know if they have coralline or macro algae or corals growing on them, nor do we know what fishes or other organisms are present in the “normally” functioning reef.
For the typical reef aquarium we have at least a few species of fish and various invertebrates present that are interacting with the nitrogen cycle end products, or just fish releasing ammonia.
The ammonia level should begin to rise and be measurable on our testing.
Yet, A tank with corals and clams only may not show any immediate signs of ammonia since they take up ammonia from the water for energy and nutrients.
A shark tank may show ammonia but the presence of ammonia is not as significant for these fishes since they have urea in their blood.
Still, for the typical reef the fish present are going to show signs of ammonia poisoning and may be surface breathing and showing general signs of distress but if the antibiotics are still effective there will be no bacterial bloom and cloudy water that we usually see when excess ammonia is present in an aquarium.
I don’t know if gut bacteria were eradicated from the treatment? If they were I would expect the fish would start showing symptoms including lack of appetite and diarrhea as well.
Uptake of alkalinity may slowdown or speed up depending on what the inverts are doing with the excess dead bacteria in the water?