There are a few reasons why an octo cant really be kept in a reef tank (correct me if i am wrong):
Reef tanks generally need alot of lightning which octos arnt partial to.
Also, Octos create alot of waste (very messy eaters
) which in turn will create nitrates. Cephs dont seem to mind nitrates to much, but in a tank with sensetive sessil inverts high nitrate levels would not be a good idea.
Furthermore, many corals have stinging cells that could damage or kill an octo.
About the trace elements and things like that they should be fine to add to octo tank water.
lastly, there is always that little thing concerning an octos need to re-decorate
. You said that this didnt bother you, but once your inverts start dying because of this im sure you will think differently.
Thats why for octos we make "species tanks" just for them.