The specialist shop I went to had these fish as temperate in 18’c unheated tanks.
Most fish stores will have an air temperature closer to 21C (70F) due to the fish tanks. An aquarium without any internal heating source will be the temperature of the surrounding air. A temp of 18C (64F) in the store seems unlikely, so the tank would have to be chilled by some form of chiller to maintain a lower temperature.
That aside, we turn to the species. Platy have a temperature range of 20-28C (68-82F). The loach,
Micronemacheilus cruciatus has a range of 25-28C (77-82F). Two things need to be mentioned.
First, the temperature range given by reliable sources for a species means the species can manage within this range, and generally mid-range long-term. The upper and lower temperatures should be considered as tolerable for temporary or brief periods, but not permanent housing temperatures for the species.
Second, stores aim to sell fish quickly; they lose money the longer the fish are there. So the environment in which the fish finds itself is almost always anything but ideal or preferable. But the fish are temporary residents and most species can usually manage to get through this. Permanent home aquaria are a very different matter; here the fish is going to spend the rest of its life in this environment, up to death. The environment must provide what the fish needs, requires and expects if we want healthy fish. Anything less and one is in the wrong hobby.
Final note on the significance of temperature and why even one degree can have consequences for the fish. Fish are ectothermic, meaning that they have very little ability to physiologically maintain (or alter) a constant body temperature. The body temperature is usually the same as the water they live in, and temperature drives the metabolism and allows the proper functioning of the fish's physiology. Each species of freshwater fish evolved over thousands of years to function best in a very specific environment. In any one environment, the fish have become acclimatized through evolution to a relatively narrow temperature range. If the temperature moves outside this narrow range for a continuous period, or if it changes rapidly within the range, then it causes stress to the fish.
Temperature changes affect the fish's metabolic rate, disturbs the rate of respiration, imbalances the pH of the blood, and causes a breakdown in osmoregulation function. For fish these are serious and compromising effects, and at the very least the fish is weakened, causing yet more stress.
