I have a 12 (UK) gallon tank, any ideas what to stock it with?

Question

I would like as many pretty/colorful fish as possible, without over crowding the tank.

Answer 1

well, do not get goldfish if you want a lot. Get small fish like neon tetras, guppies, fancy guppies, mollies, dwarf corydora catfish (these are great for keeping the bottom of the tank clean)etc. These fish need a heater too (pretty much all fish besides goldfish need a heater though).  

Answer 2

probably around 2 bettas, but that's hard because two males will fight, and both sexes will try to mate (if conditions are good).

you should get small community fish, like tetras or those neon coloured fish, maybe some mollies or danios.

just depends on what you want.

Answer 3

yellow tangs are cool

Answer 4

12 gB = 14.4 gUS

I am surprised you used gallons B instead of L, but moot.

I'd recommend 10 male guppies, assorted colors and patterns.

or 6 neon tetras and 4 cory cats of your choice.  Some of the cories have nice color and patterns, and fun to watch.

6 head and tail light or glow light tetras with 4 kuhli loaches.

6 of two different species of tetras, such as cardinals and glow light, or black neon and glow light.

1 male Betta would also be okay with the neons and cories.  These all require a heated tank.  

Dwarf gouramis are beautiful and with a few hiding place for the female you could keep a pair with tetras and cories or kuhlis.

A school of 6-8 cherry barbs with cories.

These would be some of my choices for a community of colorful fish.