95% of marine tanks are set up and maintained using synthetic salt mixes. In general most people find it cheaper and more convenient in the long run rather than depending on a store to purchase water from. It is also very convenient when you find the sudden need for a water change at 8:00 pm on saturday night not to have to go to the store to get 5 gallons of water. If you have truly bad tap water (you can get pretty good analysis of it by calling the water supply source in the city you live in - post results and I am certain somebody here can tell you if it is complete unuseable - better yet find a local
reef club and poll the members there) you may need to get an ro/di unit at some point but will probably be ok for the start-up. Use a quality dechlorinator - I like Amquel brand . The method I would recommend for you with a 40 gallon tank would be to get 2 10 gallon plastic containers. Label one for topoff water and one for saltwater. Fill them with water and a small powerhead. Use amquel (there are probably other good products available - I am saying amquel simply because I have used it many times and it is a quality product IMO) to eliminate chlorine/ammonia/chloramine etc and let aerate for a day. Add salt to one of the containers to appropriate salinity (I will not even attempt to tell you which salt is best - everybody has an opinion on it - IMO any salt mix that has been available for a long time must work or it would have gone out of business by now - I use Instant Ocean). It is preferable to let the newly made salt water aerate with powerhead for 24-48 hours prior to using it. It is probably sacreligeuos to say this here but IMO unless you are haveing real phoshate induced algae problems or are running a very sensitive
SPS tank (which you won't be with lighting you have) you most likely can manage with dechlorinated tap water quite easily. Particularly for the next 2 months or so as you cycle / cure liverock. If you have to purchase water for the curing/cycling process it will get expensive very quickly. It takes 10 minutes to mix up a container of saltwater and you just let it sit in a clost till you need it. I have a routine on sunday morning - that I will start in the next hour or two. It is a little more involved because I deal with a larger quanity of water but the process is the same....
Saturday night I plug heater into 40 gallon rubbermaid container with 25 gallons premixed saltwater in it.
Sunday morning before lights come on in tank bring container into tank. Bring extra empty container in as well.
Top off with freshwater.
Blow off liverock with turkey baster ( I have a DSB and blowing with powerhead makes a mess)
Turn off all electric to tanks ( makes me feel better and easier than unplugging every pump individually)
Siphon out 15-20 gallons water from display with natural syphon , then balance of 25 gallons from bottom of sump to get out accumulated sediment
Use mag7 to add 25 gallons of premixed saltwater back into display.
Clean skimmer and empty skimmer overflow.
Dump old water.(if you dump directly on lawn repeatedly it helps eliminate other required sunday chores like mowing grass
)
Fill now empty saltwater mixing container with 25 gallons of water , add amquel and take back to storage closet, plug in powerhead.
Make 15 gallons additional water (fresh) to use for topoffs during week.
Clean up any little spills or drips that occurred (using caution to make sure if you use the good towels to be done before wife wakes up)
Under an hour a week for 90% of basic maintenance on 200 gallon total capapacity system. BTW - I do have an RO/DI unit but since I have never really had a problem with algae I do not use it regularly (I don't like throwing away 100 gallons of water to get 25gallons)
For your particular situation you will want to be able to do 10-20 gallon water changes daily for the fitst 3-5 days as you cure liverock so plan accordingly!
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