Build looks very good so far, keep up the nice work.
As for the GFCI plug, for the majority of home circuits it will be 15 amps. If you go to your panel just find the breaker that controls that plug, it will be clearly marked for the number amps that the circuit provides.
NEVER change a breaker to higher amperage rating (e.g. 15 amps to 20 amps) without knowing what gauge wire is feeding ALL circuits. The breaker is there to protect the wire from catching fire in the event of an overload. Don't listen to some hump at Lowes or Home Depot when they say to "upgrade" the breaker, that is idiocy.
As for a dedicated circuit for the tank, that can very easy or extremely challenging depending on where the tank is located, how the home is constructed, if there are open slots on the main panel, etc, etc. I would have a qualified electrician take a look and get multiple quotes for the work. If you are comfortable with your uncle doing it, go for it, you will never regret having the peace of mind that provides. I have two dedicated 20 amps circuits for my fish room, overkill? Probably yes, however I never have to worry about what comes on when.