Lance,
I have come to like acrylic tanks for octopuses in general and especially for the mercs. The tops are usually a solid piece with an access cut out. The two inch or better top overhang serves as a good escape deterrent. Fortunately the mercs are not difficult to keep in an aquarium as long as you provide dark places for them (an all in one with an attached sump is risky and not recommended). I would not suggest anything larger than a 30 gallon tank and think a 20 is best suited for a pair. I strongly do not suggest anything smaller than a 15 and only mention one that small if you have a large, well aged piece of live rock and experience with a marine environment.
You will want to create a red light environment that will nicely light the tank (mercatoris is nocturnal and is rearly seen in any kind of white light). I have used both red velum filtering and red LED light and have had the best luck with high temperature velum placed inside the lense of an outdoor fluorescent light fixture (with holes drilled to vent heat). If you use only ambient light during the day, you will find the combination minimizes algae growth on the tank walls.
Your filtering, as with all octos, should exceed the manufacturers recommdations. I use the smaller footprint Skilter with additional air stone in addition to 5 gallon a week water and filter changes in a mature tank and have had very good results. The cascade, filtration and skimmer combination all help to keep a good supply of clean oxygenated water but weekly maintenance is a must.
Mercs tend to move around on the LR or climb the walls but don't spend a lot of time on the bottom so minimizing the bottom substrate does not negatively impact them. I use a thin layer of medium argonite sand that can easily be stirred and vacuumed (no more than an inch) and helps minimize nitrate build up.
Do note that any aquarium you set up for a merc will not be suitable for another species and if you think you may want to keep a larger animal in the future, a standard rectangular 20-30 gallonish tank makes a decent sump.
