API for everything they made kits for, except Ca. Verified their alkalinity tests by Salifert, 2 boxes - the same results, 3rd - very slightly differs (less than 1 dKH).
Tried, but don't like Seachem blue boxes kits, like Marine Basic - difficult to distinguish between colors in a shallow depth of the small concave vessels.
Seachem Reef Status (big white plastic boxes) are totally different matter, the positive side is that they have reference solutions to check, if the kit still good. The negative side - kit may stops show correct data in a several months, even being not frozed or overheated.Also quite pricey in retail LFS, cheaper online.