A fishtank is very much like a sandbox, but instead of large map with numerous dungeon at the center it’s all about a cast of interesting NPCs and competing factions for the players to clash with. In a way, this is more challenging for a GM as a dungeon with a vague origin and original purpose is muc easier done than a handful of NPCs with interesting goals and motivations. And you still have to build lairs and strongholds for them anyway.
While working on my own plans for a new fishtank campaign this winter, I noticed that I’ve actually taken a very convenient shortcut. Following the great advice of “Good artists borrow, great artists steal”, the entire main framework of my fishtank is simply the old D&D adventures Against the Cult of the Reptile God and The Elfwhisper and the excellent storyline of the Bloody Baron from The Witchet 3 all set in the same town.
All three adventures are pretty linear designs, but at the core they really are starting situations with a problem that the players are supposed to fix somehow. The D&D adventures expect the players to go to the lair and kill everything, but the videogame at least offers a good range of different linear paths to chose from. (Which only shows how bad even most better published adventures are. This is the one aspects in which videogames can not come close to the potential of RPGs.) There are really quite a lot of adventures for D&D and Pathfinder that have some really cool setups at the start. Even with a typical dull Pathfinder railroad, the initial setup is often very much salvageable and usually the best part of the adventure.
If you want to set up a small fishtank, simply grab three or two adventures that you think look cool and take the NPCs with their goals and motivations and the dungeon floorplans and put them all down on the same map. And all the heavy lifting i basically done with that. To make things a bit more interesting and complex, think a bit about how those important NPCs might know each other and how their plans might put them into conflict. Maybe add some embellishments here and there, create a handful of new NPCs and minor dungeons, and you get a decent fishtank pretty quickly.