Sorry for the late replies, for some reason fishforums.net doesn't always email notify me when somebody responds to a topic
Frustrating
I keep mine in a 29g saltwater tank. There are no freshwater octopii. As far as a reef, that is more or less impractical as octopus can't be with many kinds of corals because they will hurt the octopus... and the octopus can very easily topple corals and cover them with sand and stuff.
I keep the species 'octopus bimaculoides' so I can only give advice relating to them. They are not afraid of the light at all, as was mentioned here. In fact, mine seems to be even more apt to be out when the tank lights are on. Their native habitat is right off the california/mexico shores, about 50 ft deep or less in the ocaen. Therefore, they're used to a lot of light.
And yes with no lid the octopus will probably escape, they're very curious and sneaky. Bimac's are much less prone to this, but other octopus such as 'octopus vulgaris' is very notorious for escaping. They also get to be several times larger than bimacs, and are generally unsuitable to keep.
I feed him raw shrimp, blue legged hermit crabs, freshwater malaysian trumpet snails and freshwater clams. He is becoming less interested in the snails as he is getting larger and I suppose pickier!
His colors are very interesting. Sometimes he will be a brown/black color.. I think this is sort of his 'normal' colors. But he can turn completely white in the blink of an eye, and can turn some of his body white and some very dark. He also can turn a brown mottled looking pattern that makes him look just like the live rock. When he does this and 'hides' on the rock, he will raise bumps on his skin to blend in even more. When he gets very worked up or excited about things he will turn bright white colors with lots of black lines and orange polka dots on him. He also has a false eye spot on either side of him, a little under his eyes. These look like small bullseyes, and he can turn them dark blue and black, and can flash them. His colors change so fast, it amazes everybody that sees it! It looks like a fast pulsing, and he can change colors faster than you can blink. Very cool!!
Right now his 'armspan' is about 3+ inches, and his mantle (his head) is maybe 1 1/4 inch. He is still small, but he will reach an armspan of roughly 1 foot and his mantle will be 5-6 inches at full size.
I expected him to hide and be generally shy, but this is not the case at all. I see him out at least 6 or 8 hours a day, normally in the evening hours. I also have got him eating out of my hands with no problem. We are each learning each other still, but so far I have gotten him so used to me that he will 'shake hands' with me when I put my hand in the tank with him, and he has even got the whole way on my hand to let me take him for a ride around the tank. This friendliness really surprised me!
My camera is horrible so still photos are basically useless. I have taken video of him with my camera though, which makes it much easier to tell what is going on. Here is one picture taken from a screencap of a video, where you can easily see him. I will also link to videos of him roaming around the tank. These videos may or may not play for you..
Here is a video of him 'walking' around on the glass. You can see him change color a little bit.
Here is a video of him taking a snail from my hands and eating it. He's kind of hiding in his den so he is not very easy to see in detail in this video
Here is a picture of a very nice full grown bimac. It is clearly under blue lighting, but you can see the patterning being displayed nonetheless. This is NOT my picture --