Anirban did a good job and I will add a little more. Your question about why choose more difficult route. There is Reef salt or Probiotic salt. If you do not have a nutrient issue (No3/Po4) then you can go with the Reef salt. If you choose to fix nutrient issue or get down to ULNS then go the probiotic route. This includes Probiotic salt, -NP Pro, and Pro Bio S.
To clarify, 1+2+3+ vs ABC+K is simple. If your corals are growing at a rate that needs supplementation in addition to WC then you are handling this in one of 3 ways, manually dosing 2 part (or balling), using a doser to dose 2 part (or balling), or using a calcium reactor. What process are you currently using?
- If you are manually dosing or using a doser then you should look into using 1+2+3+. Ignore everything else. If your consumption is at a rate where you are using up the 5L quickly then you have the option of mixing your own with the dry powder and Component Strong.
- If you are using a calcium reactor then you continue using the CaRx and buy the Components ABC+Kalium.
These two processes get you through 99% of what is absolutely necessary to keep the reef going. If water changes are sufficient and you never see your calcium or alkalinity drop then stick with just the salt. If you need to add calcium or alkalinity then choose between 1+2+3+ and a doser or choose ABC+Kalium and a calcium reactor.
We can't answer whether you need to use carbon or GFO. Your tank will let you know. If your water is not crystal clear carbon will help. If you have a phosphate issue then GFO is the quickest way to remove outside of doing massive water changes.