With good filtration, you won't have to clean it everyday. Introducing new water on a daily basis may stress out the fh due temperature changes and if untreated or aged, the water may have undesired chemicals leading to more stress. This is what's taking away the color.
Once the water conditions improve, it'll be happier & healthier. I keep temperatures high at around 86 degrees. The high temp makes them more active, leading to bigger appetite and the down side, dirty water...that's why filtration is so important.
Look into sponge filters for the time being. They're simple and require little maintenance and works really well. I've kept flowerhorns in a tank with a heater, sponge filter hooked up to an air pump...and water of course...without gravel and they're happy.
Its color will come back when it's healthy.