Thats good... except the part of having to go 5 blocks to see your fish

The advantage of the basement build is that it would be cheaper to do as well as usually more space, but the advantage of the shed is that you can build it to suite your needs and you can really seal it up well for humidity and such, but you may have issues for things like Water supply (not really easy to run hot water to the shed unless you consider installing a small water heater (which doesn't always allow for large water changes etc) drainage of old water is a consideration as well since Sheds are usually placed "out of the way" it means that they are sometimes to close to garages, houses, fences ect that can't always handle a few hundred gallons of water being dumped out every few days.