Often it's a rotted pipe or a leaky hose or a leaky water pump. Usually is't best to look at it and see where the coolant is coming out, then repair it.
a gasket as a rule
When it was damaged. If you don't have coolant leakage, but the level collant drops down it means you have internal leakage of coolant. You must replace the head gasket of course if want to drive your car. van over heats
If there really is no leak then it is likely a head gasket allowing coolant to leak into the cylinder and go out the exhaust as steam.
Valvoline Zerex.
What type of coolant should i use in 2006 Nissan Quest
from a non-mechanic but usually there is some coolant leakage
yes
Pressure test the system to determine where leak is originating
You have a coolant leak somewhere. Fill and check underneath for any obvious leakage.
Not necessary to drain all coolant, just enough to be able to remove radiator hose from thermostat housing and prevent much leakage.
Sounds like a head gasket issue, or worse, your head could be cracked. Check your exhaust - that's probably where your coolant is going.
thermostst is stuck