The goldfish was replaced during one of Steven's early blackouts. Having them both there is symbolic of Marc and Steven now coexisting.
I had assumed that Gus the one-finned goldfish had died from lack of food while Marc/Steven was in Switzerland.
That said, maybe Gus and/or his one fin were just a figment of Steven's imagination - a memory of the one-finned fish his little brother had drawn on the day he died.
Maybe. Or maybe it's as simple as, "OK, who bought the OTHER goldfish?"
Hmmm... I agree that I think Gus died while Marc was out of town. How did they find another single-finned goldfish?
Also, Steven noticed time skips, and other odd things indicating Marc's activities, and the both of them have noticed the blackouts when neither one of them was in control. If they were really in control of the body, and Jake and Khonshu were still moonlighting ("moonknighting?"), I think they would notice more and more blackout incidents and get to the bottom of it.
I am wondering now if the last scene with Marc and Steven (and the goldfish) is some other dreamscape where they are kept by Khonshu now that Jake is in control. Putting them into some illusionary reality to keep them happy and out of the way seems like just the ticket.
Have I mentioned I want a second season of this?
I also don't see how Marc could balance his heart while Jake was still locked away inside. I'll call that a bit of a plot hole, like how he created Steven to shield himself from his mother's violence and yet Marc is the one who remembers it, not Steven.
Looking back at it, Taweret
only had Marc's and Steven's hearts and tried to balance them. Jake (and presumably his heart) was still locked away in the sarcophagus.
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I was also pleased to see a variation on my prediction come true, with Marc/Steven getting back in control to find all the bad guys dealt with around them.
Disney/Marvel, I really want season 2!