Fish are finding there way to peoples aquariums because they look prettyor they're on sale. If you could inform yourself with a bit of knowledge you can save lots of money and grief in the long run. Lets talk about a few different kinds of fish and what they need.
Neons: minimum of 6 in a school. love to swim near the bottom and hide in live plants, no seashell or coral decor, can be kept with other mild mannered fish like black neons, glow lights, harlequin rasboras, corycatfish, small plecos, otocinclus, will need something a little larger to keep them in a tighter school like an angel or two.
soft water, driftwood, plants
South American minimum 66 gallon tank (some can get quite large), hiding
Cichlids: places or at least plant coverage, keep with sevrums, jewels, (oscars if you must), parrots, rafael catfish, plecos, clown loaches, congo tetra and many more
soft water, plants, driftwood
African Cichlids: minimum 48 gallon tank, many many many rock caves, coral or other hard water substrate, keep with other agressive africans of similar agression, lace cats, large plecos, clown loaches, mild africans can live with some rainbows but the substrate should not be completely coral.
hard water, no large driftwood, few plants (if they don't get eaten)
Teacup Sting minimum 4' tank, large tree roots to the water surface Ray: sand substrate, keep with other like rays (obviously you'd need a bigger tank for more rays), large plecos and a few other mild medium south american fish, very timid
soft water, sand or very fine gravel, driftwood