Hi
Firstly, welcome to TONMO.com and I hope you can see that people are not ganging up on you or being negative (except to the shopkeeper) People on this forum have the animal's best interests at heart... and yours!!!
I am shocked that the animal was sold to you in this way. Unfortunetly I think its chances of survival are slim to none. Do you know any experienced marine animal keepers near you who can help or as already suggested try another store? That first store has ripped you off! taken your money and ran, but also you should have done your research first! You may have wasted money as well as an octopus' life.
If you have any other marine aquarists nearby, see if you can scrounge a few handfuls of sand from their tanks as that will be full of all the bacteria you need to get your tank going...
Nitrate of 15 is normal, in a sense. But its only there because it came in with the water, so what i mean is that it is normal for tapwater. That '15' hasnt been 'made' in the tank it came with the water. DO NOT rely on that as being an indication that your tank has cycled!!!
There wont be any nitrite and Im not surprised, its probably because it is all tied up in ammonia just now which is much more harmful and even small amounts can be fatal.
You did in one day, which I recomend should take at least 3 months (90 days).
In the Nitrogen cycle of an aquarium, ammonia is produced by the animal and is very toxic, it is gradually converted into nitrite by bacteria and then less harmfull nitrate and then either taken out by water changes or reduced even further into harmless nitrogen by more bacteria.
No matter what elixers you are putting in your tank from bottles, like many experienced aquarists I have yet to se them live up to the claims.
Do you have an ammonia test kit? get one as a priority if you dont.
If you get any reading from that at all then you need to do a water change to dilute the toxins and make sure that the new water being added is at the same pH, salinity and temperature as the tank.
Your alkalinity is fine and not worth bothering about at this stage. You have a higher reading than the shop becaus eyou have just used new salt to set up, it will diminish over time...
Being active could be a sign of stress in a new octopus! Most new octopuses will sit in the huff for a few days.
You said you spent a ton of money and sorry to say but that shopkeeper seen you coming! I wouldn'd go back there again!
I wish you the very best of luck and anything, ANYTHING, you need to know then please ask here!
cheers
Colin
