Hallo Pavel,
yes, they have their own test drops calibrated for their photometer.
But you can calibrate also other tests from other brands, for yourself of with help of the Halanek/Gabriel. If you have a known brand of test kits, I suppose you can tell them to calibrate the photometer with S.. or T .. or so.
You can also sent them your tests and let them calibrate with the photometer they will send you.
Calibrating with other test is a special service you can find in the price list.
Alas they don’t have an english version of the website.
I order the test kits from that company because its easier and even not more expansive than sera and tetra or so…