The cacti growers from Central Europe who specialize in termophilic plants have to pay
a little extra for heating. If you decide to grow the whole wide spectrum including the Caribbean species,
you have to adjust the minimum winter temperature to the species most sensitive to the cold.
Polar bears will not get used to life in Europe and M. nagyii cannot spend winter in a place where the temperature
does not rise above 15 degrees Celsius. I have risked it just once and trust me, it is not worth it.
In Cuba, the average temperature usually does not fall beneath 20 degrees Celsius and if so, it only happens for a short time. This also applies to other Brazilian and Columbian species. Just because the plant survives extreme thermal conditions once, it does not mean it can do so every year. Besides, the low temperature may cause other vegetative defects such as stains, body distortions or growth impairment.
During the growth period, the plants should have enough warmth and high air humidity (around 70-75%). Do not forget there are two main growth periods. The first one occurs in the spring in May and June and the second one in September and October (visible mass increase). During these two months, the plants should be hardened by being left near open windows but you have to watch the weather forecast since nightly temperature falls may hurt watered plants and cause loss of roots or rotting of the whole plant. During this period, we can also observe significant production of fruits that are pushed to the surface of the cephalium. We can collect these, clean them and store them in bags labelled with the plant name and year of harvest.
During the growth period, melocacti should be watered regularly (in my experience, approximately every 10 days). The flowerpot can even be left in the water. I personally recommend to add fertilizer to every other watering. The temperature rises to 35-40 degrees during the day and falls to 20 degrees at night. Such fluctuation suits the plants and helps them grow strong thorns or visibly increase the cephalium.