A few quick thoughts here. Some good info so far, and yes if you are not currently tripping the breaker you are probably okay until you add something to your setup or to something somewhere else on the circuit. Typically you will find 10-12 items on the same breaker, this can be a combination of plugs and lights in your house. As was mentioned before finding out exactly what your sharing with is a good idea. If it is mostly lights then the chances of you having an issue down the road go down significantly, and if it is more plugs then you need to be very careful what is being plugged in. In a perfect world i would recommend having new wiring run to put your system on its own circuit, mine runs on a dedicated 20A circuit, absolute overkill in my case but allows for any changes i may want in the future.

My next thought. What was the reasoning behind installing a GFCI for your tank? My guess would be water and electricity close together, lets be safe. The GFCI will do NOTHING to protect your setup, but will protect YOU if you happen to be touching anything in the area causing a small leak of power to ground. In addition running submersible pumps on a GFCI protected power source can lead to nuisance tripping, shutting down your system, hopefully someone is home and notices when it happens.

Anyway, just my two cents on the matter

9 Years working as an electrician
4 Since Journeyman Ticket
1 Since Masters Ticket
2.5 Years Reefing.