Most important thing is to adjust sg
slowly, especially if increasing it. The easiest way is to add well mixed (24+ hours with a ph) SW
for top off instead of fresh so it increases your sg
slowly over days and not all at one time.
range of 1.018-1.022 is fine. If a reef setup then keeping it closer to 1.024-1.026 is better for your stock. Slight decreases or increases is fine but if your sg
is swinging wildly then you are not topping off enough or your sg
meter is ineffective.
Swing arm hydrometers will give you the widest fluctuations IME because they are sensitive to temps and are prone to damage easily which will give you different readings each time you use it.
Floating hydrometers are slightly more accurate and the 10+ā€¯ ones are better then the smaller ones. If using a floating hydrometer do not keep it in the tank all the time since deposits that build up on it can affect the reading. Make sure you turn off all pumps so the water is steady & read the measurement from the bottom looking up.
will give you the most accurate reading and automatically adjusts to temperatures.