Ideal constant current source?
An LM317-based solution is probably the simpliest one if heatsink and extra power consumption is not a problem. It can also be turned into an advantage if you need to warm up water in your aquarium.

A better solution is to use a single-chip PWM controller, a power MOSFET and an inductor to drive the LEDs. A UC3843 and an IRF540 may suit you well.