They are actually two very different formulas, that perform two very different tasks.
API is best used to detoxify
chlorine treated tap water, as it is based on sodium thiosulfate. This is clear when you look at their SDS.
http://apifishcare.com/pdfs/product...r/tap-water-conditioner-safety-data-sheet.pdf
Also, if you read API's directions, for chloramine they simply increase the dosage, to ensure the chlorine/ammonia bond in chloramine is broken. Fine for some set ups, and smaller water changes, not a good option for those with chloramine treated tap water, especially those folks that like to perform massive water changes.
Seachem Prime & Safe are best used to treat
chloramine treated tap water, as both break the chlorine/ammonia bond, but also treat and detoxify the resulting free ammonia (NH3) that is left when that split takes place.
For those that simply have to deal with chlorine year round, then buying sodium thiosulfate in bulk is the most economical way to go. You can read more in the following sticky on this subject.
This subject came up recently in another discussion & I felt that it deserved its own thread. Hopefully it will help give some members a better understanding about water conditioners in general, and more importantly help save them some of their hard earned money. The only way that...
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HTH