Flowerhorn are susceptible to all the same diseases as other fish. Fortunately, these fish are tough, and less vulnerable. So you should not be fighting disease very often.
All of disease in Flowerhorn requires poor water for it's start.
Fish tanks which are too small, or fish tanks without adequate water changes, or filtration are the most common cause of illness.
If a Flowerhorn appears sick, you should IMMEDIATELY check the pH of the system. The pH should be above 7.0 for best results. If the pH is lower than this, the fish won't mind much, but the bacteria in the filter will not work up to par, and nitrogen will accumulate.
To keep the pH "up" you can use Baking Soda in small quantities, crushed oyster shells, or Buff It Up.
If the pH is not out of whack, you should check the Ammonia, Nitrite, and NITRATE of the system. High nitrates is a common cause of illness in established tanks. It's immune suppressive and it causes poor healing and color in fish. Here's a test kit that has everything for all the testing you need.
If the water quality is good, it still would not hurt to change some water.
Flowerhorn should have their water changed as follows:
10% per week
20% every 2 weeks
or 30% every three weeks.
Whichever suits your schedule. If you keep up your water changes, the nitrates will be down and the growth rate will be HIGH! You will also notice that the hump on your fish grows more.
Diseases can be broken into three groups, or treated with shotgun remedies.
Bacterial Infections
Bacterial infections in Flowerhorns are effectively treated with medicated food. The best one is actually for Koi and contains THREE antibiotics. It's called MediKoi. Click here.
Parasites
Ich, Trichodina, Scyphidia, Epistylis, and other ciliated protozoan diseases in Flowerhorns can be treated with salt. It's easy, safe for you, and for the fish and it works!
Their primary mode of killing would ostensibly be through the accumulation of large numbers. Especially in fish fry, the numbers of flukes encountered area accepted as the cause of mortality simply because they take such a toll on the victim when they occur in large numbers. Based on observations made in practice, however, I would like to introduce my opinion of how just a few flukes can kill a larger fish.
In the process of attacking a host, the flukes dig deeply into the epidermis and gill tissue with their happens. Regardless of species, the flukes are known to carry and inoculate pathogenic bacteria. Flukes from certain areas, and on certain batches of fish carry more and more dangerous and virulent bacteria on their hap tens. In this way, discovery of a few flukes on the gills or skin can account for rapid and mortal outbreaks of Aeromonas and Pseudomonas funrunculosis (Ulcer Disease).
Control of flukes has become increasingly easy with contemporary medicine.
source : http://frogstone-restday.blogspot.com/2009/11/flowerhorn-illness.html