I don't have any good references to cite but from my understanding, well designed LED systems only get dimmer over time (quite reasonably slowly) due to various

I don't have any good references to cite but from my understanding, well designed LED systems only get dimmer over time (quite reasonably slowly) due to various semiconductor physics. However, by far the most common failure mode for electronics including LEDs is shock and fatigue to the interconnect. This means if you let your LED thermal cycle repeatedly, even to well below datasheet maximums, the solder and bonding are quite likely to fail. I'm not sure how your aquarium lights are configured (whether they cycled or were always on) but honestly 5-7 years is already quite impressive. There's not really any other lighting technology that can do that. 5-7 years of constant usage also far exceeds what consumer batteries can usually do, which are something on the order of hundreds of "actual cycles" before significant capacity degradation. That translates to maybe a couple years, but definitely less than five.