There are misnomers surrounding aquariums and electricity.
Read this and be safe...
You should really invest in a GFCI for your lights, AND a ground probe.
Both will save you and your fish. Without both, neither will do you any good. And for most other equipment, they should also be plugged into a GFCI outlet.
Having a GFCI without a ground probe will not work, and a Ground Probe without a GFCI also will not work.
To fully understand how these circuits work, you need to read this:
Beananimals info
Scroll down to about the midway of the page and read about Grounding Probes. This section will explain about GFCI Circuits and so on.
This is a MUST READ. It could save your life.