Several months ago when I was green to fishkeeping, on my 120g FW tank I had an awful ICH outbreak.. and I decided to treat with copper. I treated for about a month, of course killing off my snails and shrimp. After all was cleaned out, I wanted to see if I could reduce my damage. I went ahead and threw a product called 'cuprasorb' into my filter, and waited for a few weeks. Since then, I have been keeping snails which breed like mad, clams, and tons of shrimp, and all of them are very alive and well.
Copper can absorb into aquarium sealant. I don't know if this happened to me or not, but if it does you could still treat continuously with cuprasorb to combat the slow leaching of copper back into the tank. Obviously getting a tank treated with copper is not your best choice, but I think this is a good solution to it.
I got this tip from reefkeepers so I assume this product works for sw and fw both. Note that I have not used this product to clean out a tank to use an octopus in, but it is my strong opinion that it would be fine.
If you need more information about this product, try to dig up posts about cuprasorb on fishforums.net, that's where I first heard about it all.
As for your other concerns,
That hole like nancy said is for overflow to occur. You want to be careful about overflows with an octopus, because they are sneaky and could end up going down this tube into your sump. That wouldn't be a good thing, for a few reasons
I imagine you could try to block it off with cheesecloth, but that could get difficult and could clog (which is even worse).
If you do use this overflow/sump return style system, you'l have to get another tank or large container such as a rubbermaid bin to hold your return pump (which you'll need to buy), and protein skimmer. Pretty much any in-sump skimmer you can buy can handle 50+ gallons. Since the tank won't be that heavily stocked, I wouldn't worry about getting the most heavy duty thing on the market. Any skimmer set to skim hard should be fine. Your pump is up to you, but I think 500 gph is what you're shooting for (around 10x turnover rate for the water per hour. I could be wrong here, been a while since I set up a tank).
Water changes are required to lower nitrates further than skimming will, and also to bring trace elements back into the water. Skimming has good effects (taking waste out of the water), but it also takes these trace elements out.. so it's even more important to reintroduce these via water changes in a system with skimming going on.
Also, to have success it is probably important that you load the tank up with live rock to use as your real filtration. Skimming, heavy water flow, and plenty of live rock is a surefire way to run a successful SW tank. In my octo and my reef tank I use slightly less than 1lb/gallon. So for a 50g I would suggest somewhere from 40-60lbs. With an octopus you want to use large pieces of rock, as larger octos could tumble small pieces and kill themselves or smash your tank wide open!
Hope this helps, if you'd like more details or help drop another note
Robert
