All previous posts were valid and true to every word. I just a few things I want to point out that weren't said (i think)
#1. The store was wrong to take the fish back for 4 days. Their system is probably crawling w/ ich and it wouldn't have gone away, and the life cycle of ich is much longer then 4 days. Which is why we QT for a weeks to a month.
#2. Understand that your canister filter will give you problems in the future. you might want to do some research on it, we've discussed it in great detail in many thrads
#3 although I believe skimmers are needed (some dont) It helps remove waste products and organics from the water if you can't do water changes as often as you'd like.
#4 Is it possible the scallop died anad you didnt notice? They don't survive in closed systems and are rather difficult to feed. If it died it could have polluted the tank.
#5 Even though your readings were normal, your test kit could be crap basically, which are you using? Also whats your SG and nitrite readings?
#6 Did you only have 2 fish in your tank? Hippo and clown? You could have brought something back with you from the store the second time around.
#7 Did you check the PH readings after you dosed? If you did to much, it could have been a critical factor
Lastely for a question posed about UV.... I find them to be very over rated. They work wonders with killing algae blooms in the water and make your water crystal clear. But they only affect organics/bactria/paracites which are in the water. once ick gets in the sand, fish and rocks. the UV doesnt work.
I suggest in the future, you stay away from ALL lfs advice and come here

Our vendors and memebers are over qualified, and very helpful! I hope you find some answers through reading posts and asking questions. And I hope my comments haven't raised to many more questions or confused you
(btw my longest post ever!)