It sounds like you are off to a good start. I feel your pain when using the Red Sea alkalinity kit. I use the same kit, and being is the light blue color zone is good. When it starts to drop below this you need to be adding a buffer. For the record, it is essential that you also test for calcium in ALL marine aquariums. Calcium is the major buffering ion in saltwater, and replenishing calcium directly is necessary to achieve the balance and stability you want from your marine system. More on that here: http://www.tropicalfishkeeping.com/m...-marine-33079/
You mention that you have some live sand and some play sand. It is likely that the play sand contains silicate. It is assumed that silicate is a cause of algae, but the hobby has had a difficult time proving that this is actually true. This being said, I hope you plan this tank to be a FOWLR tank and not a reef. The risk of using silicate based sand in a reef is probably not worth the money saved. It is a high enough risk that I would personally consider starting over today, rather than moving forward, if you plan to keep a reef. If you are going FOWLR, then I would not personally be worried about the sand. For the record, you should have a 4'' to 6'' sand depth for proper denitrification. If you are not aiming for denitrification, then keep the sand a 1'' or less to avoid problems with organic accumulation in the sand bed.
Your salinity is low. Increase it to 1.024 to 1.025. I have no idea why some LFS continue to advise keeping the salinity at levels below natural sea level. You will not gain any benefits (parasite prevention) at 1.020. All indications from experienced successful hobbyists are to keep the salinity at a natural level.
Power filter: When you say you have no media, do you mean you are running it empty? Or are you using the filter pad? If so, you should not be using the filter pad. These particulate filters trap detritus, causing organic decay and nitrate buildup.
Do you have a protein skimmer? Have you considering this purchase? We can make some good inexpensive recommendations for a smaller tank. Over the long haul, the skimmer will save you a lot of money, so you may as well buy one.
You say you have not done a water change. No big deal. I see no reason for doing a water change at this stage. You really would not be accomplishing much. Are you testing for ammonia and nitrite also? I suspect you will never see an ammonia or nitrite reading, because the live rock is established from another tank.
At this point, wait for the diatom bloom. The diatom bloom will appear as brown algae that spreads rapidly over the rocks. It will fade in a couple of weeks. You should then see coraline algae begin to take its place, assuming you are correctly testing and buffering for alkalinity and calcium. For the record, this is a good stage to add a clean up crew. For you, 2 or 3 hermit crabs and 1 snail will suffice to start.
You should also wait at this point to allow the population of copepods and amphipods to flourish. You should begin to see populations on the glass and in the sand bed. The diatom bloom passing, coraline algae growing, and pod population thriving are early signs that the system is maturing nicely and ready for livestock.
The final sign of a tank maturity will be when your nitrate levels peak and then begin to DROP. Yes, nitrates will drop back to near zero in systems that utilize a deep sand bed, live rock, and a protein skimmer. This takes a few months, but is a great indicator of long term health.
Lets talk livestock. What are you considering? Also, can we get pics of the complete setup?