I use different size for my tanks
One #1 for my 10 gallon hospital tanks
One #3 for my 20 gallon breeder tanks
One #4 for my 40 gallon tanks
Two #4 for my 55/60 gallon tanks
Planning to use two stackable #4 for my 90/100 gallon tanks
I also use #3 as a prefilter for my Emperor 400 and 280 filters.