For a serious breeder of guppies, a cull is not just a sick or genetically deformed fish though. Guppies with a color pattern tail shape, body size or shape which was not what they wanted, would probably be a cull. For American breeders who show fish, they want a match between the dorsal color and pattern and the tail.

If the fish don't have that, they may not make the cut. Some people will feed them to larger fish. Some will dump them at a local pet shop (one man's cull is somebody else's community fish). Some people just flush them. :0

That sounds pretty brutal. Fancy guppies are not what "nature" decided a guppy should be though and one must be very selective in caring for their strain(s) or the strains can drift away from what is desired. (Once was told maybe 5 of 5,000 goldfish from a spawn are show material. Similar issues exist with Bettas too.

A look at the judging form of a guppy group will indicate what the competitors must breed for if they want to win.

General clubs often have a 100 point system where 1-20 points is awarded to a fish on the basis of size, color, finage, health and department – for the species. It is a lot harder figuring out deportment for a bristlenose catfish than a guppy. ;)

The World Guppy Club (as per their Brazilian show in 2003) has the following judging scheme:
http://www.january04.guppylabs.info/

LENGTH     SHAPE        COLOR         POINTS
BODY        8        8        12        =28
DORSAL    5        8        10        =23
CAUDAL    10        20         4     =44
VITALITY/DEPORTMENT    =5
Total    =100

It is interesting to me that different national guppy clubs often have a different vision of what a good guppy in a strain is. They may also be more enthusiastic about different strains.

I still find myself looking at some fishes and thinking, "I don't know what art is. But I sure know what I like."

There is a flip side too. If guppies were raising and showing humans, I'd have been culled some time ago. ;)

There's a lot more on what to keep and what to cull. If you really want to get into it, you may want to start with the references below.

All the best!
Unc;e

The following offer suggestions as to what breeders are trying to develop with different strains.
http://www.ifga.org/articles/1articles.htm

http://www.ifga.org/articles/breeding10.htm

This German site offered European standards. Interestingly females awere not regularly judge in 1997. Males were shown in groups of three:
http://www.aquaworldnet.com/awmag/a_3gupen.htm

This is a thread from the Singapore guppy group. As you can see, they are grappling with what should constitute a class and show the fish should be judged.
http://sgguppy.com/index.php?board=25;action=display;threadid=246

Here is an overview of what Luke Roebuck feels are appropriate proportions for different strains of guppies:
http://www.january04.guppylabs.info/

One of Midge Hill's articles on what to select for when breeding (and culling)
http://members.tripod.com/~PPGA/guppygambitart.html

IFGA classes:
http://www.netpets.com/fish/reference/freshref/classesgup.html

Drawings showing what is desirable:
http://guppysa2z.com/clubs/sjgg/tail03.htm

If you have a breeding programme for your fish, you may want to cull the deformed or those that have had deformed fry, the slow growers, the ones that don't conform to your colour standards/shape/size or the ones who seem prone to disease.

That's how I understand it anyway.