I hope noone gets offended by all those jokes.
My first tank consisted of a 3 liter wine bottle full of tap water and housing a single red sword fish. I changed all of the water every few days by pouring it in the sink trying to not loose the fish in the process.
The glass was pale yellow, thick, and uneven. At the age of 9 I felt like the best aquarist ever.
Later a 1/2" sunfish was added to the same bottle. It was captured in a pond situated in the middle of the gypsy slums. I didn't venture to go there because such a visit usually meant a fight with the gypsy kids involving rocks flying from all directions. My friend that didn't like to think much about anything went there and got about 30 baby sunfish. I was fascinated by the "wildcaught" fish and by the picturesque explanation how the sewage from the houses formed aromatic ditches and filled the pond where the fish lived.
Later I heard about guppies living and breeding in colorful chemical waste pits somewhere in the former USSR. Some Russian tanks seem to get inspiration from sights like that
Years later I saw wild cichlids in Africa. The fish occupied almost all the free space in the lake's water. Roughly one fish per gallon of water, many species, and all possible sizes.
Oh I also recall seeing wild sailfin mollies in Cuba, in a part of a river running into the sea (obviously brackish).
What were your first experiences with aquariums?