ahel, Im not telling you what to do, but If I were you, I would consider at least some aragonite sand mixed in with the play sand if you still want to keep the play sand.
I was taught that the marine environment needs to draw from the aragonite to maintain some balances.
Another thing important to me is the refugium filter and a good skimmer.
Seems the refugium on a 24/7 lit cycle will keep your tank without pH swings and keep oxygen at saturation levels saving you from expensive pH buffers and sudden fish deaths. The skimmer will help remove wastes.
Try to find some good seeded live sand and macros to kick start your filter. If you don't find any, I will help you with some.
Salt water is not hard if you use common sense. There is a cycling period.
If your using artificial salts, read up on the toxicity of marine salts in the reefkeeping magazine right here at RC and make up your own mind.
Of course, this is only my oppinion and you can use this info anyway you like.
I made the BIG mistake of converting my freshawater discus/stingray tank into a saltwater reef and re-using the same rock and substrate. Please don't make the same mistake. What I learned the hard way was that the rocks and substrate absorb some of the chemicals used in the freshwater tank,ie-Iron and plant fertilizers and release them once they are absent -like in the presence of a saltwater reef tank. These chemicals are harmful to a reef environment. This process will happen very slowly and not show up right away. What this means is that your tank will seem established and fine but slowly become toxic from things like phosphates or chemicals. My freshwater tank was fertilized and established for plants etc, that is why I noticed such a problem when these chemicals re-released into the saltwter environment. If you haven't used any freshawter only chemicals in your set-up you might take a chance and re-use your sand. I personally wouldn't do it from experience but that is just my 2-cents worth.
I converted a freswater tank (pirahna) to a saltwater ceph tank. However, I did not reuse any of the substrate or rocks. I also sterilized the tank before cycling it for 4 months in prep for the octo. Considering how sensitive cephalopods are to water contamination, I say better safe than sorry.
I sterilized using a solution of maybe 15% houshold bleach in water. After doing that I used a product like Amquel etc to neutrilize the chlorine. (This can be risky if you leave any trace of chlorine in the tank.) I rinse the tank mult times after that.
I have done this after Ich, or other, outbreaks and have not had any problems. It is not necessary that you do this and a good scrubbing and rinsing of the tank will most likely suffice.