White cloud or green cloud?
White cloudiness is from free floating bacteria--usually not the same as the nitrifying bacteria. They usually die off as soon as the food source is gone. Increased water changes, and reduced bio-load (ie, feed less) will help.
Green cloud means algae. More water changes, less food, monitor light period, and adding plants are your best bets.
Flowerhorns get big--while it is okay at 3" in a 10 gallon, this a large bio-load, and that could be part of the problem. You will need to upgrade tanks, and these fish are often very fast growers. If yours is not, I would suspect stunting.