The sand turning purple is more than likely cyano-bacteria which isn't huge, but it's a sign of new tank syndrome which everyone goes through. The spots are parasites, my guess is you didn't quarantine and the tank is going through mini-cycles (even though your numbers read zero or acceptable levels) which in turn stress the fish lowering their immunity. Now, stress doesn't kill fish, but it leaves them more susceptible much like if you go without proper rest or eating you catch a cold or flu. Stress in my opinion is a constant, we can never reproduce oceanic conditions, not that your fish will always be stressed, just that at any given moment throughout the day, temp or ph flux will cause some stress. So, the only control we have, is to eliminate the parasites. Which can only be done in a quarantine tank, there are thousands of posts and stickies on this forum that will go into way more detail than we have time here to do, as there are various treatment options, various parasites to treat. Start there, and if you have any other questions you can pm me, or post it here, there are a lot of smart very experienced people that check this forum on a daily basis. And I am not at all putting myself in the category of the smart ones. Anything I know, or think I know, it's because I am on here everyday reading and researching and applying technics. My best advice is to do two things in either order. Get yourself a Quarantine tank: 10-30g nothing fancy. And read and research everything you possibly can to help you in the long run saving money on this very expensive addiction. You will be rewarded a Thousand fold if you do.
