There are very few fish that will live happily in a 15 ltr tank. The livebearers mentioned in an earlier post are active fish who need room to swim around: guppies need at least 10 gallons, mollies more like 20-30 gallons. Most other fish are the same, they need space. Also, most fish produce too much waste and need more oxygen than such a small tank can provide. And a small tank is going to be a less stable environment, with less reliable water stats, so not such a good option for neons, which are sensitive to water stats.
One exception is the betta (Siamese fighting fish) mentioned above. It is a slow moving fish, so does not need a lot of room, it can breathe oxygen to some extent from the surface, and it does not require company (indeed you can't keep two males together, they'll rip each other apart). One male betta would be a good option for this tank. And we have a betta forum specially for you, to advise on setting him up!
Other options might be a couple of African Dwarf Frogs (NOT African Clawed Frogs) and/or some freshwater shrimps.
Caddicam, there are various methods of working out how many cms/inches of fish you can keep. A common one is the 1 inch per gallon rule, so one inch of fish per (US) gallon of water. Note that this is the full stocking once the tank is established.
But most importantly, it is only the very first of a long line of rules:
i. It only applies to slimbodied fish that do not exceed 3 inches. You can't stick a 10 inch oscar in a 10 gallon tank and expect him to survive.
ii. It does not apply to heavy waste producers (e.g. plecos)
iii. Fast swimmers need a long enough tank; a 10 gallon is not a good solution for five 2 inch danios.
iv. Tall fish like angels need tall tanks.
v. Schooling fish need to be in schools.
vi. Some fish need more space/big tanks/to be kept on their own because they are territorial.
vii. Only a very few fish can live in tanks under 5 gallons.
viii. Unless you do a fishless cycle, the fish have to be added very slowly, starting with no more than 1 inch/5 gallons and starting with the hardiest.
ix. Some fish are predators. Neons are the angels' natural food in the wild.
x. Some fish are fin-nippers. Tiger barbs and guppies do not go well together.
Alex: a betta is a good choice for a 4 gal. A goldfish is a very bad choice. Goldfish (if treated decently) grow BIG and produce a lot of waste and need a lot of oxygen. The recommended size is 20 gallon for your first common goldfish (with fancies you can get away with 10), and then 10 more for each additional fish. With the proper space, they will thrive and grow into the magnificent fish they deserve to be.
