1. You could use the 2 x 15 amp breaker. If you only use one side of it it will still trip like normal. Chances are you will not need to use this though since you have 4 other circuits to use.
(see below)
2. All this means is you have two seperate circuits both 110V. One being the red wire and the other being the black wire. They just happen to share the same neutral. If one circuit trips for some reason the other will as well because the breaker is connected.
3. This would be the same as number 2. Only difference being is that if one trips the other will not because the breakers are not connected. The green wire most likely is a ground. Check if it goes to a ground bar in the panel.
Sounds like you will have lots of dedicated circuits for your fish room.
Id come help you out but its a bit of a drive for me...
