i dont think that such a small spike would necessarily kill the octopus. Im not sure. But if i was going to keep an octopus, i would keep it in a 125g + tank, which probably would have eliminated the spike alltogether cause thats more than double the water volume.
Besides that, it looks like you did a good job. Im sorry to hear about your loss, better luck next time. Dont be afraid to try again some time. Let the tank mature more, and get a bigger tank, and i think you would be ok.
But anyways, im sorry to hear of the loss.