I should have thought to put a disclaimer at the beginning of my new journal thread (New 8-armed buddy). More or less it is an experiment to see what will and what won't work out. There is a small long-tentacle anemone in the tank, as well as an LPS coral, both capable of stinging. That said, I've been maintaining various saltwater tanks for over 2 years, almost 3 years now, so I have a pretty good feel for how these things work.
Personally, the best advice anyone can give you being your first saltwater experience, is keep it simple. Be VERY patient, and do LOTS of research on EVERYTHING you consider BEFORE purchasing. The biggest mistake a lot of people make is being impatient, cutting corners, and impulse buying things without any knowledge beforehand of the care and behavior of the creatures they take home.
As for octo-proofing your overflow goes, I added an extension of pvc onto the overflow drain. It is 8 inches long, and has LOTS of 1/8th inch holes drilled in it, the complete length of the pipe, as many holes as I could fit, so as to not cause drag/slow down the drain flow. The end of the pvc has an end cap. Depending on the size of the octopus this should work well, but I haven't heard of anyone else doing it... kind of a last minute idea, and I'm happy with the way it's working.
As for other things you can add to an octo tank, gorgonias and soft corals are pretty safe. Any LPS is going to involve risk, and should most likely be avoided, especially a beginner. To elaborate, most soft corals do fine in low lighting conditions, and do not have sweeper tentacles that can sting. Octopuses don't seem to like bright lighting, which LPS and SPS corals need. So, the addition of these type corals is going to require more intense lighting, which will in turn most likely cause the octopus to hide, plus octopuses have sensitive skin and contact with stingy corals can be unhealthy if not lethal. Serpent/Brittle stars and urchins are tride and true safe tankmates. If you're interested in fish, you should get familiar with their behaviors/personalities before getting too set on the idea of adding any. A good place to start would be fish labeled "peaceful" and suggested for a "nano" tank. These fish will generally be passive and considerably small, more or less incapable of threatening an octopus.
My "project" is to conclude whether or not an A. aculeatus is suitable to a typical fish/reef-tank setup. This may turn out a success, and it may turn out a disaster. I hate that I am subjecting the octopus to potential harm, but at the same time I am considering that the octopuses natural environment is significantly more dangerous. I wouldn't have jumped ship on the species-only rule had I not read several possitive experiences from various people on various boards, and felt comfortable with my own knowledge of the creatures included in the tank with the octopus.
Until I conclude that specificly an A. aculeatus is capable of surviving, and as best I can tell, thriving in this tank, I do not endorse keeping one with potentially hazardous tankmates. Another octopus to consider for this type of experiment would probably be an O. briareus, but in a larger tank since they do grow much larger than an A. aculeatus and would need more room to avoid trouble.
Just because Evil Knievil jumps through hoops of fire and across the Grand Canyon on a motorcycle doesn't mean others should follow, to put it one way. I could have kept my endeavor a secret to avoid any damage to my tree-hugging reputation but I wanted the experience, and I wanted to share it, and I wanted to learn from it and share what I learn from it so as to hopefully expand on the possibilities of octopus keeping. Having my thread pointed out brought to my attention that I am maybe setting a bad example for future octopus keepers, and I don't want that at all.