Rainwater is a pretty terrible solution, if you needed to implement it at a large scale to deal with water supply issues then you'd easily be within a decade, and probably a few years, of absolute disaster. Rainwater collection reduces the amount of water going into the local aquifer and in sufficiently urban areas this can lead to ground instability while also depriving down-shed areas from having water access - though I'd mention that rainwater collection as a way to simplify the water infrastructure isn't a terrible idea especially in remote communities since it can remove the need to support local water extraction and some of the piping. Still, care needs to be taken to ensure water safety with rooftop collection.
That'd work in the tank (besides the fish excretions) but in a wet system, you have standing water in downpipes the diameter of a tennis ball that might get a little restrictive for fish.
You'd need all intakes (e.g., downpipes from gutters) to be protected with mesh, the tank itself to have likewise, but the most painful bit is handling those intakes/overflows getting blocked by leaves/debris. My pipes get blocked by leaves even without mesh. There are a variety of angled mesh systems you can use but adding them to each down-pipe gets expensive.
it's an externality that can't be hand-waved away as trivial when considering massive adoption of rain barrels.