The main differences are output and flexibility. The 580 is bigger, puts out more light, is a little more adjustable and can be used to trigger wireless canon flashes. The 430 cannot be used as a master flash (to trigger other flashes), but can be used as a slave flash( be triggered remotely by the 580).
Now I'm not very familiar with Canon gear, but I think you will need a transmitter to fire either flash off camera or you will need a cable to connect the flash to your camera, so you can have the flash above the tank.
Nikon has the ability built in to fire flashes remotely. I use a transmitter, but it's not necessary.
If you find the means to trigger your flash wirelessly, and there are ways, then the 430 would do just fine for what you're doing. One way os to buy Canon's transmitter. Another is to buy radio transmitters like Pocket Wizards. Unfortunately a set of Pocket Wizards cost as much as the 580 flash, lol.
Lucky for you, you can buy knock off transmitter/receivers very similar to the Pocket Wizards for a tenth of the price. I'll go see if I can find a set and price. Basically, the transmitter goes in the camera hot shoe and the receiver attaches to your flash. A radio signal firs the flash remotely.