During a long journey, a fish continues to breathe and produce waste. Because the transport bag is sealed, carbon dioxide from respiration accumulates in the water. Some of this dissolved carbon dioxide reacts with water to form weak carbonic acid, lowering the pH of the transport water. At the same time, nitrogenous waste causes total ammonia to accumulate. Ammonia exists in water in an equilibrium between ionised ammonium, which is much less toxic to fish, and unionised ammonia, which is considerably more toxic. At the lower pH that develops in a sealed transport bag, the balance is strongly shifted towards ammonium. Once the bag is opened, accumulated carbon dioxide begins to escape into the atmosphere. As dissolved carbon dioxide falls, the carbonic-acid equilibrium shifts back and the pH tends to rise. The size and speed of that rise depend partly on the water’s alkalinity, or buffering capacity, and on the rate of gas exchange. As pH rises, the ammonia equilibrium shifts in the opposite direction, increasing the proportion present as toxic unionised ammonia. This creates an acclimatisation dilemma. Gradually adding clean aquarium water dilutes the total ammonia in the transport water, but it may also increase its pH. During a prolonged acclimatisation, a fish can therefore remain exposed to accumulated transport waste while an increasing proportion of that ammonia becomes more toxic. The opposite risk is also real. Fish should not be moved abruptly between substantially different temperatures or water conditions, and sudden changes in water chemistry can themselves cause stress. The safest approach depends on balancing these two risks rather than applying one method mechanically to every fish. Published guidance illustrates this trade-off. OATA’s betta care guidance is framed around a fish being taken home promptly from a retailer and recommends a short floating-bag acclimatisation, including the gradual addition of aquarium water. By contrast, specialist ornamental-fish shipping guidance published by Cole and colleagues describes fish typically arriving after 24 to 48 hours in sealed bags. It recommends matching temperature while the bag remains sealed and then transferring the fish directly into the receiving water. The same guidance warns that opening the bag allows accumulated carbon dioxide to escape, causing pH to rise and potentially increasing the toxic fraction of ammonia while the fish remains in the transport water. These recommendations were developed in different settings, and neither proves that one method is universally correct. They do, however, demonstrate why journey length and transport-water condition matter when deciding which risk deserves greatest weight. For most fish arriving after a long courier journey, where the receiving aquarium is fully cycled and its temperature and water parameters are appropriate for the species, we favour temperature matching while the transport bag remains sealed, followed by prompt transfer into clean aquarium water. This minimises unnecessary time in opened, ageing transport water. It is also worth remembering that the pH measured in an arrival bag may be lower than the water in which the fish was normally kept, because of carbon dioxide accumulated during transport. Arrival-bag pH therefore does not necessarily represent the fish’s normal pre-transport water conditions. The underlying transport chemistry is well established, and published studies document deterioration of transport water with increasing journey time, including in Betta splendens. Specialist ornamental-fish guidance has also recognised prompt transfer after prolonged shipment, while conventional guidance continues to recommend gradual acclimatisation in other circumstances. What remains limited is direct controlled research comparing those receiving methods after next-day courier transport in species such as bettas and channa. Our recommendations therefore combine established chemistry, relevant transport research, published husbandry guidance and practical experience rather than claiming that one method is universally correct.