Because octopuses are different from other marine creatures and saltwater aquariums barely resemble freshwater tanks, we try to encourage new keepers to use best practice methods of aging a tank and acclimating the animal. We do lose some animals even under the best conditions but starting out with known setups helps minimize losses.
Since you are starting again, would you entertain reconsidering your tank environment? For ANY marine setup, even though your water was tested with no ammonia and no nitrates (I will assume that is what you meant by "perfect"), that does not tell you the tank is cycled (often the pronouncement of fish stores who are unaware that no biological process was ever started). Water never exposed to decaying biological matter will show as "perfect" until you add biologics. If you used dead rock, you will not have had the bacteria needed to start a tank cycle. You will need to add something to begin the process. Live rock, dead shrimp and pure ammonia are three options (live hardy fish are also used but are undesirable on a number of levels). Live rock is my preferred method but look up cycling with the other methods to decide which will work best for you.
A newly cycled tank (one that has shown a spike in ammonia followed by a spike in nitrites with both eventually registering 0 - roughly a month later) will be the first step in preparing a tank for your octopus but still is not ready for its intended occupant. After the initial cycle, the tank needs to continue building bacteria so that when waste is produced, it will immediately be converted and not poison the occupants. This extended cycle takes about 2 more months of adding a few clean up crew critters and overfeeding them (there are other methods of building the bacteria but this is the simplest). You can add snails, crabs, hermit crabs and/or serpent stars and urchins to feed. These can be left in the tank and become food (in the case of the crabs and sometimes the snails) as well as permanent residence (ignored snails and hermits and permanent tank mates serpent and urchin).
Introducing your octopus to a cycled tank should take about 3 hours. The goal is to S L O W L Y match PH, temperature and salinity. The method I like best is to half fill a bucket with tank water (adding new saltwater to replace the water you removed from the tank) and then placing the BAGGED octopus into the water. Using another container of tank water, I use a turkey baster to add the tank water to the bag at roughly a turkey baster full every 10 minutes to start and 5 minutes near the end. Once the bag is full, use the baster to drain off half the water and continue the process. I highly recommend getting a set of test strips that will show you PH (testing salinity with the water you take off is also a good idea). Test your tank water and (using a separate strip) your octopus water and do not introduce the animal until they match.