Ideally you want to keep the water parameters as stable as possible. This isn't as important in a FO or FOWLR tank because the fish can tolerate relatively high nitrate levels, but if you have invertebrates or corals in a reef setup, then fluctuations can cause the demise of the inhabitants. Keeping tabs on your water parameters is essential in that instance and will dictate when a water change is needed or supplements added.
Your SG will fluctuate as water evapourates from the tank. It is only water that evapourates, the salt is left behind and so the concentration gets higher, thereby raising the SG of the tank. As the water level drops, you need to top up with RO water, not salt water.
When performing a water change and removing 20 %, you replace with the same volume of water at the same SG. The frequency of this will again depend on what your water parameters are (nitrate buildup etc). If your parameters are stable and within acceptable levels for the tank inhabitants, then its not going to matter too much if you do one big one, once a month or 4 small ones a month. In this instance, water changes will be mainly about replacing trace elements as opposed to removing unwanted organic matter dissolved in the water.
Some of the parameters that are monitored (but wont necessarily apply in all tank setups) are :
Temperature
Specific Gravity
PH
Alkalinity
Ammonia
Nitrite
Nitrate
Phosphate
Calcium
There are others too, which escape me at the moment. Obviously if you have a FO tank then most of these wont apply. If you have a FOWLR and inverts, more will apply. If you have a reef setup, all these and more will apply.
Hope that goes a little way to explaining.
AK