Livebearers are definitely weak from a genetic perspective. Line breeding by the fish farmers in Asia is the main cause. Line breeding is simply breeding the same bloodline of fish over and over again without adding new bloodlines. This causes problems in any animal or fish.
This all goes back to the 1950s when Livebearers started to be made available to the market. The fish in shops today look nothing like their wild counterparts so to get more colour many fish were hybridised. Guppies were crossed with Endler's Livebearers, different species of platy were crossed with each other and with swordtails, different species of swordtail were crossed with each other and with platies, and different species of molly were crossed with each other. This was all done to add colour to the otherwise dull looking fishes.
Once a nicely coloured fish appeared, the fish farmers inbred the colourful fish to each other and got more colours. Once they had the colours they stopped hybridising species and simply inbred parents with offspring and brothers with sisters to keep increasing colour and patterns.
If a new colour or pattern emerged then that fish and any of its relatives that looked similar, were used to create and establish that colour or pattern. Again this was done by inbreeding brothers with sisters and parents with young.
About 20 years ago livebearers had deteriorated to such a degree, a number of Australian importers were thinking about dropping them because they had so many health issues, primarily caused by inbreeding. The fish farmers were told to shape up or we would buy our fish from somewhere else. Because of this the fish farmers in Asia have been adding wild caught bloodlines to try and fix the problems. Unfortunately for them most livebearers were protected and it is illegal to collect many of them. Some people were given permits and others just went out and caught them and smuggled them out of the country. Eventually new bloodlines were added to established colour forms and the fish are better than they have been, but they are still nowhere near as strong (from a genetic perspective) as they use to be or should be.
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Combining the genetic issues caused by 50+ years of inbreeding with breeding in the same ponds in Asia that have been incorrectly medicated for 50+ years, (causing drug resistant organisms and leaving chemical residue in the ponds to weaken the fish), and major overstocking issues in fish farms, importers and retail tanks, and you have a group of fish that really don't get a very good chance to do anything except die before their time.
If you can get locally bred stock they are usually much healthier and better quality than imported stock. And if you breed your own, the fry will usually be healthier than the parents especially if you cross the different colour forms of the same species.
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I must point out that inbreeding and genetically weak fish is not restricted to livebearers. Goldfish are extremely inbred and are just as crap when it comes to health issues and defects. The original goldfish is a long slender dark bronze coloured fish with a single short tail. Today's goldfish still throw out a portion of the fry as bronze. These bronze fish get sold as feeder goldfish because they aren't colourful. However, they grow faster and have few health issues than the more colourful fish with weird body shapes.
Fantail goldfish and fish with short fat bodies have numerous health issues and most of them are caused by shortening the fish's body to create short high bodied fish. Unfortunately when you take a fish and squish it up, you not only damage the spine and cause deformities to the back, but you also squish up the intestines & stomach, the swim bladder, liver and the kidneys. These organs then have more trouble working and the fish have more internal problems. And the organs being squished up can put more pressure on the heart.
Most fish including normal wild bronze goldfish have a fairly straight digestive tract starting with the mouth and running almost straight along the body to the anus. When you make a short bodied goldfish you squash the guts up so the food no longer flows normally along a straight intestine and problems arise with air being trapped in pockets in the intestine, and kinks and bends occur causing major issues to the fish.
Squishing up the swim bladder causes problems as well and 99% of goldfish that have swim bladder issues, are the short bodied varieties like fantails, moors, pearlscales and orandas.
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Goldfish are not the only fish to have short deformed bodies. Balloon Mollies have the same issues as short bodied goldfish, and Dwarf Gouramis have numerous health issues caused by massive inbreeding to get more colourful fish.
Off the fish topic for a moment and short snouted dogs like cavalier spaniels, bull dogs, pugs & boxers have breathing issues caused by being inbred and getting a shorter snout. And Persian cats have breathing problems too caused by shortening the nose/ snout area. People have screwed up lots of animals not just fish, and they claim it makes the breed better. I can't say what I want to about those breeders because I will get kicked from the forum. But basically, if an animal, bird of fish is inbred too many times, then problems will arise and the animal's life will normally be shortened due to the inbreeding for specific traits.
