cadre;178621 said:
I found an acrylic tank that I may try to buy if my current equipment ever sells. It's about 75 gallons and it has a false wall on the back that I can use for filtration and all that fun stuff. I won't be adding a sump because of the water damage issue in this apartment but it is drilled so I can consider it in the future. Sound a bit better than the 55g?
You're probably talking about a "uni-quarium". Because the back compartment was (probably) designed to hold bio-balls, most uni-quariums were used for fresh water, so there's a higher chance that a used uni-quarium was exposed to fish medication containing copper. Copper is so toxic to octopus that most of us are afraid to use a tank that has ever been exposed to copper, for fear that the acrylic or silicone (or glass?) may leech enough copper into the water to kill an octopus, so try to verify that copper medication was never used. The fear of used tanks that were exposed to copper may be very legitimate, but it may also be exaggerated or unfounded. When an octopus dies for no visible reason, after living in a 2nd hand tank, copper is suspected, but I don't know if anyone has ever deliberately put an octopus in a tank that they know had been exposed to copper, to test the theory (ethical issues). So try to verify that no fish medication has ever been used in that tank, and if it has, try to verify that it didn't contain copper.
I agree with what D said about acrylic tanks. There's also no greenish tint as there is with thick glass. Another difference is that the bottom of an acrylic tank must rest on a strong, rigid, flat surface, where as a glass tank only needs to be supported on the edges, so be sure to get a stand with a flat top.
Oh, remember that the volume stated for the uni-quarium is sort of a lie, because they use the outside dimensions of the whole tank, including the rear "sump" section, to calculate the stated volume. so if the outside dimensions are 48"x18"x20", they would call that "75 gal". At 231 cubic inches per gallon, and assuming 3/8" thick acrylic, and a 3" wide rear "sump" area, the main tank will only hold (47.25 * 13.875 * 18.75)/231 = 53.2 gallons. Plus whatever water is in the rear section, minus the volume taken by your sand and live rock, so call it about 53 gallons of water net. That's how much water is in my bimac system (also no sump (to help keep heat out)) and the species you plan to keep are a bit smaller than bimacs, so you should be okay if you don't load the tank up with live rock, or stinging anemones that take up space the octopus could be walking around in. I still like the idea of partitioning off the end/side of a rectangular tank, giving the octopus a closer to square living area, rather than a long narrow area, but if it falls in your lap for cheap, it doesn't make sense to modify an existing tank yourself.
As far as leak protection goes, I think you're smart to want to avoid a sump if you've got downstairs neighbors. Acrylic won't break like glass can, so you'll be close to zero chance of a leak. Short of an earthquake tipping the whole thing over, I think you'll be covered.