There are dozens of different ways to secure a tank and I have tried to list a few of the more complete threads in the
Tank Buildouts sticky at the top of the Tank Talk forum. The best configuration I have found is to use an acrylic surround that is about 2" wide and sits in the small lip of a framed aquarium. Unframed tanks are a bit more difficult but you can achieve the same effect by gluing support rods on the inside of the tank, creating a support lip. (I also use acrylic rods for this and find that the 3M automotive double stick tape works reasonably well as long as the surfaces are VERY clean and VERY dry and left alone for 24 hours). A somewhat complicated version of the arrangement is shown in my going larger thread
here. The lids over the access holes are also acrylic and recessed to be flat with the top (overkill for a merc tank but esthetically pleasing) by attaching thin acrylic to the underside of the surround at the openings to support covers cut to fit the openings. We drill our tops (but not the surround) using a peg board as a template, time consuming but again, they look nice. The holes are to help release heat but serve no other purpose.
If you use acrylic for any of the top, take the time to sand
all exposed edges smooth. This is not for the octopus, it is for your hands and arms. Cut acrylic is VERY sharp and the 30 minutes it takes to sand it makes all the difference in the world and is easy to do with simple sand paper (it also looks nicer, just keep the sand paper to the edge and avoid the clear surface).
CuddllyCuddlefish, if you are aware of glass with vent holes, please reference it as I have never seen anything that would be appropriate.
Most common clean up crews are acceptable. Snails and hermits will likely be reduced in numbers initially but most octos prefer the foods we offer and will leave them alone once they learn feeding time. I have read of mercs eating one or the other of these but have never had mine to eat any of them (the larger ones have but have never completely emptied the tank - with the exception of Little Bit
). Any serpent star (I would avoid the greens though as they become aggressive at some point in their lives) or brittle star will be fine. After a tank is fully mature (about a year) I particularly like the thorny sea stars because they are diurnal and often brightly colored and do well in an octo tank (I can keep them for several years without problems in spite of reading otherwise). Start slowly with the CUC, adding the hardiest first (hemits and snails) and slowly work toward adding any that are more water sensative and need algae (stars and cucumbers). You can also include pencil urchins in the CUC and they do a nice job cleaning off the rock but be aware that they will also attack gorgonians and sometimes soft corals if they don't find enough to eat (Kara has seen them eat the wood in their holding tanks).
After your tank is mature, sponges and gorgonians can be added for color. Gorgonians need to be placed where the octo does not normally traffic. Octopuses don't go around things, they just amble over them. This is more of a concern for the larger species than for mercs though. I also occassionally add rooted macro algeas but they don't survive long and are often eaten by the clean up crew (one reason I add them). A few low stinging polyps will work but it is hard to know which will and will not affect the octopus. If you add polyps, attach them to a rock of their own and be prepared to remove it if you see the octo jerk away. Mushrooms and leathers are fine as well once the tank will support them.
Provide loose shells and light pieces of coral rabble for the mercs. They will use these as doorways to close off their dens when they sleep and sometimes when they eat. Finding an out of place shell will often lead you to a den if you can't find the merc.
Cycling and continuing a cycle once established is the topic of many forums and threads, the following is my personal recommendation and hopefully others will chime in with their own. If you are using live rock (my choice 100%) to cycle your tank, once you see a spike and then reach the zero nitrite, zero ammonia stage (or in 2 months if you see no spike), start adding your hearty cleanup crew and over feeding the tank. Keep your bottom substrate clean though as you do not want to build up nitrates. Many people add fish at this point, I do not and don't see a need. I recommend not keeping any fish with any octopus, regardless of some of the successes you may read. There are a number of reasons for this, the most important one being that most fish have been treated by the suppliers and if the octo should eat it, it may kill the octopus. Never feed it live freshwater fish as that is almost guaranteed death. If you want something swimming around, order a quantity of shore shrimp. They are a good food for the ceph (when they can catch them), can be left in the tank and will increase the bacteria when they die.