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In order to get the maximum efficiency and output from any solar device (swimming pool, water or space heating, cookers, etc), it should be pointed at the proper elevation toward the sun. Since the vertical position of the sun changes throughout the year, so should the tilt of your solar device.
The base to start with is the latitude angle of your region. The farther north you are, the lower the sun is in the sky. Therefore, in a northern area at a high latitude location, the glass surface should face lower than in the south. Also, the sun is highest in the sky on June 21 and the lowest in the sky on December 21. In spring and early summer, the glass surface should face up at your region's latitude angle minus 20 degrees. This is the angle that the glass surface makes with the ground, not the direction it faces up.
For good year-round performance, like for a solar water heater, tilt it at your latitude angle. For winter space heating, the glass should be tilted at your latitude angle plus 20 degrees. This faces it lower in the sky. The tilt for a solar swimming pool heater is not as critical because they are usually relatively low efficiency and low-cost anyway.