GFCIs and AFCIs share some similarities but are distinctly different.
GFCIs simply detect current flowing to ground instead of back to the neutral (white) wire. i.e. current flowing outside of the usual path. They will generally trip within 25 msec if >5 mA of current is going to ground instead of to neutral. They are available as circuit breakers, as outlets, or as external devices as in an extension cord, as was suggested above. They are required in bathrooms, kitchens, garages, and other 'wet' locations. Thus the general recommendation that they be used for aquariums. Their primary purpose is to prevent shocks that may injure or kill people. They do not detect arcing, nor do they detect excess current, as long as it is not a ground fault.
AFCI's look for current arcing either between hot and neutral or hot and ground. As circuit breakers, they trip for excess current (i.e. 15 or 20A.) Most (? all) AFCIs will also detect ground faults, but they do so at higher current levels and more slowly than GFCIs and are not suited for personal protection. Their primary purpose is to prevent potential fires caused by arcing wires, not to protect people. They are now required in most living areas as was mentioned previously in the thread.
Early AFCIs were very prone to nuisance tripping, which I believe is what Bean was getting at; a nuisance trip is more than a nuisance for an aquarium setup. My personal experience is that the newer AFCI breakers are much more stable an less likely to trip. We live in a 2 year old house and have yet to experience a nuisance trip from an AFCI. I cannot speak in general, however.
Alas, this thread has wandered far from the OPs original question and is getting close to a bickering match, so perhaps it's best if we all let it rest. If he/she has more questions, the OP can start a new thread.