I would honestly give it a full month before adding any more shrimp, and probably do a small water change prior to adding them (a few days before) only using marine salt and RO water.
Also keep an eye on the temperature, being on this side of the world it can get pretty cold.
The only way to remove contamination or poisoning is to do a water change (not 100% water change) or do a few smaller ones over a few days.
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