when you say large tank what are you implying?
Like a 90
gal? Or a 45
gal?
This is what I do.
I have two 30
gal trashcans. 1 is for freshwater storage and the other is for saltwater mixing. I mix saltwater in batches of 20-25
gal. I have a heater and powerhead in the trashcan. I mix at laeast 1 day in advance but usualy i have saltwater made up.
The 'lazy' way is to get a large pump and hose and pump the water from the storage container into the tank once the dirty water has been removed. Else you just use 5
gal buckets and scoop out water from the trashcan and into the tank once the ditry water has been removed.
Doing a 10% water change biweekly is not going to take you more than 10-15min even on a 90
gal tank. Its two 5
gal buckets out and two 5
gal buckets in.