Don't take this the wrong way, but you really should've done more research before getting that Octo right now.
First off, 1.020 is too low
. You will need to increase the salinity to 1.025 or 1.026. Being inverts, they need FULL STRENGTH sea water.
Secondly, DO NOT feed them feeder goldfish
. These contain much higher levels of fat than what they eat in the wild and it will die of organ failure if goldfish are its main diet. Ghost shrimp and baby clams (live) are fine when they're small. You can wean them onto frozen food later, however live is always best plus having to catch it gives them something to do and helps them not to get bored. You can feed him hermit/fiddler/etc crabs when he gets a little bigger.
Thirdly, what species do you have?
Did the fish store say? Is it a common octopus (vulgaris), a bimac (bimaculoides), etc? Knowing what you have is important to how to take care of it.
Fourthly, do you have lots of rocks or ABS/PVC pipe connectors for him to hide and play in?
Fifthly, octos are extemely intelligent, they get bored easily
in small tanks. Make sure you give him new things to play with/look at from time to time (like plastic lego blocks)
Sixthly, keep the lid closed and put a heavy book on it if you have to.
Octos are really strong as well as being able to squeeze through the tiniest of holes. If it can get its beak through an opening, it can get its whole body through it.
, since you mentioned it was aqua-cultured, it's probably a bimac, you will need to get him out of that 10 gallon pretty quickly, as they grow to a decent size (although not huge). However, make sure that 30 gallon is cycled properly first, they need a mature tank.
Eigthly, they require more oxygen than fish and carry a heavier bio-load
, roughly 3 times that of a fish the same size. Do you have enough filtration/skimming to handle it?
Lastly, did I mention you should've done a little more research first?
Octopuses are too intelligent of a creature to bring home and have it die in a month because you didn't know what you were doing (no offense).