For what they cost now IMO
you should not be without one.
Your tap water may be fine but IMO
I would not trust it.
Lets see... Tank depending on size 300-500, skimmer 100+, lights 200++ depending on type size, LR
$8 a pound in some places, LS
... Fish some $4 some $40++
can be bought for $100+ depending on GPD.
Its a no brainer if you ask me.
I know people who used tap and were sorry they did... Months of fighting algae and then buying a ro
and doing water changes for months till they got the water quality to stop algae growth.