First, you must have a tank that itself should consist of a 70 gallon water capacity or more. Sometimes a 70 gallon may not be enough, so you may consider upgrading to 100 gallon.
Flowerhorn grows very fast which is about 1 inch every month. Due to their rapid growth that helps them avoid being swallow by other larger fish. Due the big size they can reach, Flowerhorn need big aquarium with a lot of free space for swimming; a 200 liters tank is the minimum you can offer to an adult fish to let it live healthily.
It is a really strong fish and can live in different water conditions without having problems, anyway it is important to provide a temperature between 25°C (77°F) and 30°C (86°F); pH value is also important, because acid water can tone down fish colours and make it sick, the ideal condition is a light alkaline water, with a pH between 7 and 8. Moreover it is necessary to avoid sudden condition changes of temperature and chemical values, because they can make Flowerhorn being more sensible to sickness like bacteria attacks.