The CupriSorb is an interesting resin but I have no experience with it. The quick check I did came up with no negative reviews and a lot of evidence that it helps remove copper from tanks treated with copper medications (never do this to a saltwater display tank). Levels of removal are not quantified at the low values believed to be necessary but I believe it is worth a try. You will need to find a VERY fine bag (I believe it also comes prebagged) for the product as the grains are TINY. Any bag that suggests it will hold Purigen will have a fine enough mesh. If you see it turning blue or tinged with blue you will know it is working but you will also know your copper levels are very dangerous.
Saltwater is not topped off but removed and replaced. You will need a siphon hose, preferably self priming (
something like this), a 5 gallon pail
with a handle (remember this will weigh about 40 pounds. Good exercise but you may want to consider dumping it halfway through the process), a turkey baster (yes,
kitchen appliance that looks like a tube with a rubber bulb at one end). After your initial cycle you will start your water changes. Once you have an octopus, you will need to do them more frequently (once a week recommended and 5 gallons is about 10% and the minimum - I usually do 10 gallons a week on a 65 gallon+ sump system with an octo in residence. For comparisons to other discussions rank your tank as a heavily stocked 30 gallon fish tank and not a single occupant species only 50 with sump - octopuses are messy). When you siphon off your water, you will want to attempt to remove debris as well as water. Fan/blow on the rock (with a water moving pump, turkey baster or by hand) to move settled waste into the water column and use you siphon to capture what you can ( this will also help it move to your primary filter). I also vacuum and disrupt the bottom substrate (sand) at this time.
Another grocery store item to pickup (in addition to the turkey baster) that proves invaluable for many things - feeding an octopus among them - is a pack of
bamboo skewers. 100 will cost you somewhere between one and two dollars and will last a long time.
Note that the links are for visual reference only, these are grocery store/dollar store items with no special requirements (turkey baster should be glass or plastic, avoid metals).