The reason you should use calibration fluid is because the refractometers on the market for marine aquariums are almost exclusively brine refractometers, not seawater refractometers. Brine refractometers don't "see" all the salts in seawater, they only "see" a couple salts. As long as a brine refractometer is calibrated at whichever salinity you want to use, it will be accurate at that salinity only. If you're testing, say, hyposalinity with a brine refractometer calibrated to 35 ppt (1.0264 sg), it won't be accurate at 11 ppt (1.009 sg) for hypo just like it isn't accurate at 0 ppt for RO/DI or distilled water.
Most of the confusion lies in the instructions for brine refractometers which say to use distilled water because if you're testing brine with a brine refractometer then distilled water is a perfectly fine calibration solution to use. Most marine hobby brine refractometers are rebranded, such as Vertex, and they don't bother to put proper instructions for seawater calibration in the packages.
D-D and Red Sea both sell true seawater refractometers. The best ones on the market are made by VeeGee and cost about $160. If you have a true seawater refractometer then distilled wTer can be used to calibrate. You can also calibrate with seawater solution and achieve a dual point calibration which allows you to double check the calibration solution.
Calibration solution is a whole other can of worms...I've noticed they vary by as much as 5 ppt from one brand to the next. I started making my own (Randy Holmes Farley published a recipe, Google it he also explains why brine refractometers read off when calibrated with distilled water) and bought dropper bottles off Amazon (like ones used in test kits or for eye drops) to store it in so that there is no contamination or evaporation. I've found the Two Little Fishies solution to be the most accurate, however it's in a large open top bottle that's prone to evaporation/contamination very quickly even when the lid seems closed well.
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