PWCs will replenish your depleted trace elements. They are VERY important. Don't wait till you have problems to do a PWC
. I like to premix mine for at least 24 hours. This is my schedule:
Sat-test water/add salt/water if needed
Sun-remove old water/clean equip in old SW
/pump new water into tank
This premixing will allow you to adjust your premixed water to your main's parameters, especially pH, salinity and temp.
There are more critters in your tank that you can't see and water that is out of whack can cause even more problems. It is also a great to practice measuring salt/water combo, especially if you need emergency PWCs. I would also invest in a QT. I would hate to see that 135 gal
get wiped out by ich or poor water qualities.