It is commonly seen, when ich first starts up, that all of the trophonts are on the same cycle - they drop off the fish to form tomonts at the same time, then the tomonts release theronts at the same time that cause new trophonts. Eventually, the population gets out of sync with each other and spots appear all the time, in greater numbers.
The only other thing that produces spots that come and go rapidly are sand grains sticking to the fish. This does not look like that to me, and tangs do not have sticky skin mucus that tends to cause sand to stick.
I've never used Sera Costapur F, but it is malachite green and formalin. I have used that combination in other products and it is not very effective against Cryptocaryon, marine ich. Coppersafe or Hyposalinity are the two best treatments.
Ich management techniques:
1) Use a strong UV sterilizer
2) Perform frequent water changes
3) Siphoning the sand late every night to remove tomonts
4) Good mechanical filtration to remove theronts
5) Low dose peroxide additions
6) Provide a good diet
Ich management only works if you catch the infection early enough and do it right. Once the number of trophonts on the fish reach a certain point (perhaps 30 or so spots on any one fish - and yours may be past that point) then "propagule pressure" comes into play, where the effects of the trophonts themselves stresses the fish and the ich management techniques start to fail.
Jay