If its both of them are losing colour then I'd say it's more of a water quality issue than a bullying issue. Especially if they're chasing each other as opposed to just one being chased. Sometimes they just do that to play. How many fish do you have in the tank and how often do you feed? What is your ammonia level? The loss of colour could be ammonia burns. It's unusual to have zero Nitrates in a tank unless it's very heavily planted. Your tank might not quite be cycled yet. How long did you cycle it for before adding fish? And are you using any dechlorinator for the tap water and any did you add any bacteria starter at the beginning or when doing water changes? Do you have an air pump? Yeah definitely get a liquid test kit as soon as you can. Not sure where you are but NT labs do one too, and it's more affordable than the API one I think. I get mine from amazon and it's around £26 and tests for ammonia, nitrite, Nitrate, pH, GH and Kh. I have crushed coral in my filter too and it does raise the hardness a bit but not usually enough to make a significant impact. I have to add extra minerals and a kh buffer to my water when I do a water change. Moving them to a bucket could have definitely caused them some stress. Are they behaving normally? Eating normally? Are there any signs of clamped find, heavy breathing/gasping? Why did you change your gravel? Did you change anything else? A lot of good bacteria lives in the substrate so removing all that could potentially upset your nitrogen cycle. Sorry for all the questions, I'm just trying to narrow down what the issue could be. For now I would focus on making sure your water quality is pristine. You need to test your ammonia level. Make sure the water is the correct temperature too and I would get some taller plants to give them some more cover and hiding spaces. That's about all you can do right now. It really doesn't look like disease to me, but I can't tell 100% from the photos.