Culling is a necessary evil of raising any fish.  Even the most successful of breeders will have to cull fish with deformities.  This is best done as early as possible.  It is a waste of tank space and resources to raise fish that will be culled later on.  Most fish that need to be culled have deformities that were due to poor environmental conditions while the fish was developing.  Conditions that lead to problems are usually related to either a high ammonia level or dirty conditions leading to an increased bacterial content of the water and coating the bottom of the tank.  To ensure the least waste of resources and the best chance of normal development for the rest of the fry, those with deformities should be removed as soon as possible.  The smaller the fish, the more difficult it is to detect deformities requiring culling.  Large fish with defects are easy to spot but you will be benefited with better quality fish if you cull as early as possible.

The most common deformities requiring culling are:

Below is a group of older fry that have likely suffered from overcrowding.  The dorsal, anal and ventral fins all show signs of bacterial damage.  Given improved conditions the dorsal and anal fins might improve but the ventrals, especially on the fish where they are really short will not re-grow normally if at all.

Although most defects are environmental in cause, some are definitely genetic in origin.  One known genetic defect is missing ventrals either one or both associated with pearlscale angels.  Definitely known in marble and gold pearlscale lines, this defect appears to be recessive in nature although its exact inheritance has not been worked out.  Two normal appearing adult pearlscale angels can produce these fry usually in about 25% of the spawn.  This would suggest that both parents carried the gene and each passed it onto half of their fry.  It is difficult to eradicate unless fish known to produce the trait or actually show the trait are crossed to test subjects to see if they carry the gene.  Unless careful selection is done, the trait will likely be carried in a line only to appear several generations later when two fish that carry the trait are crossed.

Culling Methods

The preferred method of culling fish would be to use the culls as a food source.  In other words, feeding the culls to larger fish or other  animals.  While this may sound harsh to some it should be remembered that this happens in nature all the time.  In the wild very few angels reach adulthood.  Most are eaten by larger fish or other predators.  It is nature's way of ensuring that only the strongest will survive to propagate the species.

If culls can be identified early enough they can be fed to adult angels and provide an excellent source of nutrition.  Larger culls will have to be fed to larger fish or other animals.  Many hobbyists keep oscars just for such purposes.

Another widespread culling method that has been used for many years is to place the fish in a bag or small container and put it in the refrigerator or freezer.  As the water temperature drops the fish's metabolism slows down until the fish dies.  It is considered to be a humane and painless death for the fish.

Another method that is used is to place ice cubes in water and allow the water to get very cold.  The fish are then placed in the water and will expire in a few seconds.  It is also considered humane because of how quickly the fish expire.  It is important to keep adding ice to the water so it is not allowed to warm.  If the water is not ice cold it will take longer for the fish to die which is not as humane.

Sedative type drugs can also be used to cull fish.  One such formula is 1 cc of pure clove oil to 9 cc's of vodka mixed with one gallon of water.  With this method the fish is basically sedated to death.  It takes approximately 10 minutes for a quarter size angel to expire.  The smell of clove oil will permeate any bucket with the smell as well as anything it comes in contact with including your skin so caution should be exercised.

While there are many other culling methods available, the Standards Committee suggests you find the most humane method you feel comfortable with.  Flushing them down the toilet is not recommended nor are some of the other more brutal methods.

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1. Gill Plate Deformities:

This one shows both a shortened gill plate and damaged ventral fins.

An adult gold pearlscale male with a small defect in it's gill plate.

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2.  Ventral Deformities:

This one is missing it's ventrals entirely and should have been culled a while ago:

A pea sized fish showing a rearward facing ventral fin. 
This is also occasionally seen in large numbers as the only defect in a group of fry:

Below a young gold pearlscale angel with only one ventral.
  Many in this spawn had either one or both ventrals missing while the rest of the fish were normal:

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3. Anal Fin Deformities:

This deformity is likely due to an injury while the fish was much smaller.  Needless to say, the anal fin will not grow back :)

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4. Belly Sliders:

A group of fry that turned into belly sliders late in development.  Some insult affected most of the group at the same time.  The defect is believed to be in the swim bladder, probably due to a bacterial infection.  Once this develops, very few if any will recover, even with antibacterial therapy, and it is better to cull the group.

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All of these deformities are usually caused by environmental problems and are rarely genetic in origin.  The 2 most common are shortened gill plates and deformed ventral fins.  Interestingly it is common to get a group of fish with just shortened gill plates and another group raised in a different tank with damaged or missing ventral fins.  It is not clear why this occurs and you do not always get both deformities together.  Perhaps it is a different insult that causes the two different defects or perhaps it is simply the timing of the poor conditions that determines what part of the fish is damaged.