I can't recall anyone making their own packs out of them on here. I've seen several people use car batteries, but not single cells. I've thought about it a few times, but other projects have always taken precedence.
I have seen people use solar panels to trickle charge the batteries, but I'm not sure it is all that efficient.
I have not seen these types of batteries used before for an aquarium backup power supply. Most of the DIY battery backups I have seen use larger battery strings. I am sure its possible to use them its just a matter of making sure you can build a battery pack with enough capacity to run equipment needed during a power outage.
As always, be careful with 18650 secondary lithiums. Always use a pack protector, and always use appropriate chargers. Since weight and volume isn't usually a big concern, it can be cheaper to just use sealed / AGM style lead acid batteries, plus they're a lot safer and charging is a lot easier (can be floated). Its also easy to interface 1-2 12V cells in series for most DC power heads, where a 12-24V series string of 18650s will require a more advanced setup and cell balancing.