Tap water

I feel your pain. My first tank was just like yours. Every month I had to pull my rocks out and scrub them with a toothbrush. (It wasn't live rock. This was 17 years ago.) The hair algae was so long my clowns would host a patch. When we moved I never set it back up again because I was so discouraged. I didn't use RO water back then.

When I set up my new tank two years ago I bought an RO/DI water first. My tank has never had tap water in it and I have never had hair algae growing on my rocks. Unless your tapwater is pristine I wouldn't even attempt saltwater without an RO filter.

 

first off if you intend on keeping sps EVER you will have a hard time with it if you are using tap water. With this being said, your water is no better off than city water that is treated. Most of the time tds is going to be higher due to the lack of filtration. Most of the time, city water does undergo some sort of filtration getting rid of a little bit of the nastiness. Any time I see someone saying that they have always used tap water and see pics of their systems, it's always got some sort of nuisance algae whether it be bubble algae, hair algae or something else.

 

+1
I would never put tap water in my tank.

 

I have a 500+ gallon system in rochester ny, with nothing BUT tap water. I have an ro unit and i never use it. I think it really depends on the area, any one who has seen my tank will tell you how pristine it is. No one blieves i use tap water,i have a house in my basement to top off evap, and BIG D, i keep about 50+ sps in my tank RO is the way to go for most, but i have been lucky and am spared that expense. So, yes it can be done BUT looking by your luck id get a RO unit asap