There is a lot of unreliable information on the web.
I have seen an adult briareus kept in a home aquarium, and there is no way it would fit into a 15 gallon aquarium. You need plenty of room for it to swim, extend its arms, and also to take care of the amount of waste it will produce. The arms of this octopus can be two feet long, so it needs some room!
Also, I had a briareus hatchling grow up in my 19 gallon aquarium, and it was time to move her to a larger aquarium after 4 or 5 months (unfortunately, she died before the move). If you start with a smaller aquarium, you need to have a larger aquarium cycled and ready for the move - it takes too long to start one up as an afterthought.
So, at least a 50 gallon aquarium for a briareus, the bigger the better.
Nancy