They have the contest you mention during fish club meetings. You might have a fish club in your area. Here is a link to locations of fish clubs from around the world.
There are different clubs around, some are based around tropical or marine fish in general and others are aimed solely at a specific types i.e. Discus, Livebearers, Bettas etc..
My dad used to breed and show guppies many years ago, when there were good quality specimins around and he attended many show in London and europe.
If you live in the U.K. then read on.......
Speaking from experience,(Bradford and District A.S.) and one who was into the whole scene of fish shows etc (trophies and certificates to prove)
You really need to join you local Aquarist Society where you will gain a whole lot more insight to showing, judging standards, fish classes etc. They will usually host an Annual Open Show, open to all and they will have inter-society shows whereby they compete with another society. Points are awarded for 1st, 2nd, & 3rd and at the end of the evening these are added up to see which society has won. (all fish are judged to a specific standard by recognised qualified judges)
Below is a list of the Aquarist Societies up and down the U.K. http/www.practicalfishkeeping.co.uk/pfk/...b_directory.php
Regards
BigC