You can make a DIY test fluid to verify it here:
Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm
A 3.714 weight percent sodium chloride solution matches 35 ppt (sg = 1.0264) seawater for use with a hydrometer.
To produce a 3.714 weight percent sodium chloride solution, dissolve 1 teaspoon (6.20 grams) of Morton's Iodized Salt in 161 mL (161 g) of fresh water (making a total volume of about 163 mL after dissolution of the salt). This solution can be scaled up as desired.
For a rougher measurement in the absence of an accurate water volume measurement:
1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1½ teaspoon of salt (making about 82.4 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified fresh water (about 2104.4 g)
4. Add an additional 2 tablespoons of purified fresh water (about 30 g)
5. Dissolve the total salt (82.4 g) in the total water volume (2134.4 g) to make an approximately 3.7 weight percent solution of NaCl. The volume of this solution is larger than the Coke bottle, so dissolve it in another container.
How to Use a Specific Gravity Standard
Depending on the type of hydrometer, one would use this solution differently.
For standard floating hydrometers (Figure 2), which are
not self-correcting for temperature variations, it is important to use the standard at the same temperature at which the aquarium water will be tested (within say, ± 0.5 ºC or ± 1 ºF). Preferably, that will also be the temperature at which the hydrometer is intended to be used (often marked on it), but that is not an absolute requirement. The aquarist can then mark on the hydrometer the level to which it rises (that is, the water line), and use that as an indication of the specific gravity of S=35 seawater, which has all of the properties listed in Table 1(specific gravity = 1.0264, etc). If the hydrometer reads higher or lower than 1.0264, then the aquarist can just imagine the scale on the hydrometer to be shifted
self-correcting for temperaturevariations, add the standard to the swing arm hydrometer at roughly the same temperature at which the aquarium water will be tested (say, ± 5ºC or ± 10ºF). Once the reading stabilizes, the aquarist can mark the reading (or just remember it) and use that as an indication of the specific gravity of S=35 seawater, which has all of the properties listed in Table 1 (specific gravity = 1.0264, etc). If the hydrometer reads higher or lower than 1.0264, then the aquarist can just imagine the scale on the hydrometer to be shifted up or down, and shift all other readings taken with it by the same amount, just as for a standard floating hydrometer.
Just to be especially clear: this solution need not be used at exactly 20ºC (68ºF). It will be just about as accurate at 25ºC (77ºF) since specific gravity does not change much with temperature, and these salt solutions would be expected to change density with temperature in about the same fashion as seawater. The most important factor is that the temperature of the standard, when measured, be the same as the aquarium water when it is measured..
How to Use a Standard Hydrometer
Here are a few additional tips for using a hydrometer:
1. Make sure that the hydrometer is completely clean (no salt deposits) and that the part of the hydrometer above the water line is dry. Tossing it in so it sinks deeply and then bobs to the surface will leave water on the exposed part that will weigh down the hydrometer and give a falsely low specific gravity reading. Salt deposits above the water line will have the same effect. If any deposits won't easily dissolve, try washing it in dilute acid (such as vinegar or diluted muriatic acid).
2. Make sure that there are no air bubbles attached to the hydrometer. These will help buoy the hydrometer and yield a falsely high specific gravity reading.
3. Make sure that the hydrometer is the same temperature as the water (and preferably the air).
4. Read the hydrometer at the plane of the water's surface, not along the meniscus (Figure 2; the meniscus is the lip of water that either rises up along the shaft of the hydrometer, or curves down into the water, depending on the hydrophobicity of the hydrometer).
5. Rinse with purified freshwater after use to reduce deposits.
6. Do not leave the hydrometer floating around in the tank between uses. If left in the aquarium, deposits may form that will be difficult to remove.
How to Use a Swing Arm Hydrometer
In addition to those described above, here are some special tips for swing arm hydrometers:
7. Make sure that the hydrometer is completely level. A slight tilt to either side will change the reading.
8. Some swing arm hydrometers recommend "seasoning" the needle by filling it with water for 24 hours prior to use. This presumably permits the water absorbed into the plastic to reach equilibrium. In the case of the hydrometer that I tested in a previous article, the hydrometer became slightly less accurate after "seasoning."
