No why? I’m just trying to find the best fish for a 3 gallon tank
The 'why' came from your apparent readiness to dispose of an animal that had been inappropriately placed. I've responded to your new Goldfish-related thread.
Smaller and smaller tanks became more and more popular amongst people who would not normally have had room for a 'proper' tank and the commercial side of fishkeeping saw an opportunity and seized it with both hands.
You can now browse the internet and see all sorts of recommendations for these tiny tanks, but few, if any, tell you about the actual inappropriateness of the fish placed in them.
Plus, smaller tanks are much harder to maintain than larger ones, because their water chemistry is so much more easily upset, with death and sickness and overall stress to the poor creatures within.
Guppies and Endlers are often suggested, but these would have to be all males, or all females, because when livebearers breed, they do so with great gusto and a 3G would become quickly overstocked.
Betta are also often suggested, but this is usually based upon the idiotic myth that these fish normally live in muddy puddles, so a 3 gallon would be luxury.
Fish-wise, at a push, I might recommend chili rasbora for a 3 gallon. These are tiny and colourful, but need company. That said, quite a few could live in a 3G.
Shrimp and snails are other options, but note that, as has already been mentioned, shrimp need a fairly well established tank, with regards biofilm. I note your tank has already been 'matured' , so this might not be an issue.
