Due the big size they can reach, Flowerhorn need big aquarium with a lot of free space for swimming; a 200 litres 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 and 30°C; pHvalue is also important, because acid water can tone down fish colours and make it sick, the ideal condition is a lightalkaline 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.
Water filtration is really important because this big fish produces lots of refuses that end increasing nitrite and nitrate levels in the water. You can choose both internal and external filtration, even if the second one is preferred to leave more free space to the fish. The biological part must work properly, so be sure to have a well activated aquarium before to house a Flowerhorn cichlid.
Take also a look to the water current, an excessively strong one can damage the fish, anyway it is important that a slow movement is present, because it oxygenates the water, and avoids that the water heats only near heater.
Tank decorations are important to make the fish feel quieter. Use a layer of fine gravel and be sure that rocks and woods are stable and do not risk to fall easily on the fish; be sure, as well, that all the decorations do not make the tank cleaning too much difficult. Live plants are important for filtration and oxygenation, anyway choose strong plants like big anubias, because any Flowerhorn cichlid use to dig a lot; you can also use plastic plants, even if they are not useful and you can risk that the fish accidentally eat them.
Being big and aggressive is better not to house it with other fish species, specially if they are smaller. While if you plan to house more than one Flowerhorn, provide a big tank and divide it with accessories to let fish divide the territory; to avoid fights it is recommended not to keep more than two or three fish together in the same tank.
source : http://frogstone-restday.blogspot.com/2009/11/flowerhorn-aquarium-set-up.html