what kind of octo do you have? my bimac just passed..... we had him for almost 9 months....
as far as urchins, i would say definately not.... if your octo gets pricked by him, they will usually bite their legs off( watched mine touch a fuzzy shroom, and freeaked out and then bit off his arm).... octo's have very delicate skin..... once they tear or bite their leg off, they become VERY suseptable to infection.... also if it is a pigmy be careful of with larger hermits, as they can pinch the legs of your octo.... also if you need more info there is a website dedicated to octo's..... forgive me if this is illegal here, and delete if needed... the website is www.tonmo.com they are very willing to help and very knowledgable...
good luck
shane
do you have any idea what ocean he/she came from..... a "common" octo is just an octopus they dont take the time to try to identify.... if you can post a pic and what ocean it is from i could help you to identify what species, as some will require extreme conditions(cold water, different feeding habit, etc)....
as far as the lighting, i would go to a 10k or even 65k as the blue spectrum can damage their eyes. as far as night veiwing, they make red nocturnal lights by current, and red is the best cuz octo's dont see the red....
be sure to give them a good hiding place(a den) with a large conk shell or rocks to build their own.... also do regular water changes of about 20% once a week.... these guys are really dirty and the trace elements are important to them...
also make sure to run carbon all the time cuz if for some reason they ink it is noxious to other animals in the tank and the carbon will be there to absorb the toxins... if you happen to be there use a shrimp net, and yo can net it out...
if there are anymore questions, feel free to pm me and ask, i love these little guys, and am always willing to help...
i have rescued 3 from the lfs that had no idea what they had, and drove them to my buddy in washington (he researches the metabolism of them) so they can live ot the rest of their lives happy and well cared for...
shane