Frequency of water changes will depend on many variables:
Type of filtration
Bio-load
Circulation (rate of overturn)
Amount of light
Most people change out about 10-15% once a week. There are no set guidelines, and no perect frequency. Testing the water for pH, alkalinity, Nitrates, calcium and other parameters will give you more specific answer than guesswork.
You will need:
-1 cheap powerhead...like a rio 800 or a smaller maxi jet
-Rubber tubing that will fit over the powerhead outlet
-1 cheap heater around 100-150 watt
-1 cheap air pump and stone (optional)
-1
refractometer (or hydrometer that has been verified)
-5 gal or more bucket (s) for the mixing-salt mix
-good source water-no tap water unless its been tested
-a notepad to track all of your tests
1. Fill bucket of any size with the water, place powerhead in and run it. Then add appropriate salt for that amount. Only add the salt to the water, never add water to the salt. Add slowly.
2. Check the salinity (or Specific gravity) in a few hours. Ad more salt (or fresh water)
3 . Cover and let circulate for around 24 hrs. A couple hours before you will use the replacement water, add the heater. Test salinity (or SG) after water gets to targeted temperature, which for most is around 80. But acceptable range is around 77-82 give or take, varying opinions from many people. Test pH and alkalinity as well. This is your best indication of whether or not the wate ris ready for use.
4. As well, before the change, check the tank water for level. If evaporation has occured, now is the best time to add fresh water top off so it may circulate for a while.
5. Test salinity again. If this is good, I suggest testing this water for ph, alkalinity, calcium and magnesium. This way you know the shortcomings of the mix. DO NOT ADD ANYHING TO THE REPLACEMENT WATER TO ADJUST except salt. It may cause a preciptaion event, which i learned the hard way one day. If salinity is too high or too low, either add a little more salt or just unsalted water to adjust. If you must do this, let sit for a little while longer and circulate. Test salinity again.
6. Ready for the change? Take the pump head out from the bucket, attach the rubber tubing, put the powerhead in the tank, and pump out into another appropriate bucket (or outside the window ;-) and drain the same amount out of the tank as you have mixed to add back in (this is why "topping off" before hand is important-it will give you a good baseline)
Use the same pump in the bucket to pump the water back up into the tank from the bucket, to fill the tank.
7. Wait a few hours or so, then test your tank water. Given what you may know is lacking in your replacement water from the tests on it, plus the test on your tank water after replacement, this will give you a good idea what is needed.
Track these results. It will give you a good basis for next time.
Remember...always test the salt mix, and always test you tank water before ever adding anything to it. And make changes slowly over time.
Do not rush parameters to targets, and don't try and go fo the "perfect" numbers game. SMost times it is better to keep less than perfect numbers consistent, than to be erratic.