In terms of certain aquarium fish which now exist only or largely in the aquarium trade, such as red-tailed sharks or cherry barbs - this has nothing to do with conservation but exactly the opposite. The reason they are gone from the wild is because they have been over-fished for the aquarium trade. (But there are, of course, some species - especially of cichlids - which were deliberately taken into captivity for conservation reasons).
The following is a comment to the above dealing with the status of certain freshwater fish vs. aquarium trade. It is only indirectly related to the subject of this thread.
If not interested, skip ahead to the final section below the last line.
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It can be discussed if captive breeding for maintaining a population in the aquarium trade counts as "saving". However, neither of the above are examples of species that 1) Are extinct in the wild. 2) The primary cause of decline was capture for the aquarium trade.
The cherry barb remains reasonably common in the wild. Although capture for the aquarium trade has played a role, the main cause of its decline is habitat destruction. There are other Sri Lankan freshwater fish where capture for the aquarium trade appears to play a greater part (e.g. Ceylon snakehead, fire rasbora complex), but even in those habitat destructions takes the leading role. Unfortunately, with the current plans of dams on the island, it is entirely possible that the endemic freshwater fish (+freshwater crabs+freshwater snails) will follow the route already taken by their frogs. Sri Lanka's frogs pretty much hold the world record for most species extinct compare to overall species number.
Although once feared extinct in the wild, a tiny remaining wild population of the red-tailed black shark was discovered a few years ago. As noted by the IUCN and Kottelat, there is no evidence that capture for the aqurium trade played a major role in the general disappearance. Rather, it is restricted to Chao Phraya, one of the most affected rivers in the world with dams, water extraction for irrigation and pollution (unsurprising, as it flows through Bangkok). Many species that are of no interest in fishing or the aquarium trade have disappeared from its main course, but fortunately most of those survive in other rivers. However, two near-endemics are all gone: Balantiocheilos ambusticauda and Platytropius siamensis.
So, the above are not examples of species where capture for the aquarium trade appears to have played a major role. However, there are other examples, e.g. Siamese tigerfish, Banggai cardinalfish and red-lined torpedo barb, where the aquarium trade has played a greater role (although even in those, other factors are involved). In the last two species, large-scale captive breeding has removed much of the pressure, but the Siamese tigerfish continues to edge towards complete extinction and it has never been captive bred. Little is known about the status of Myanmar's (Burma's) freshwater fauna in general, but there is also potential cause of concern for some of the restricted range species that regularly turn up in the aquarium trade, but never (e.g. clouded archerfish, humphead glassfish) or only infrequently are bred in captivity (e.g. sawbwa barb). The appearance in the aquarium trade of Chinese cavefish, Sinocyclocheilus and Triplophysa, is also a cause of concern. Tiny numbers, but you don't need much when dealing with species often restricted to a single cave system.
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Freshwater fish species that only survive because of conservation dedicated captive breeding belong in three groups:
Lake Victoria (and its smaller satellite lakes) cichlids.
Pupfish.
Splitfins.
However, some of the Lake Victoria cichlids have proven difficult to maintain, as they tend to revert to the generalist morphotype when kept long-term in captivity (i.e., the specialized forms disappear). In addition to the few species from the above three groups that are extinct in the wild, several species from them are also maintained as "insurance" should they disappear entirely from the wild. The same is the case for a number of Cameroon lake cichlids, Congo rapids cichlids, Madagascar cichlids, Madagascar rainbowfish, New Guinea rainbowfish, suckermouth catfish (time will tell if the zebra pleco can survive in the wild after the Belo Monte Dam opening), Somali+Oman cavefish (although their limited captive breeding makes it questionable if this is feasible), Sewellia monolobata, Sulawesi ricefish, Limia livebearers, etc. If any of those match this threads <500 individuals (wild only?) rule is unclear, but I suspect not. However, some are still far more threatened than many larger animals with less than 500 individuals. They often have tiny highly specialized ranges where a single event easily can wipe out the entire species.
If also accepting species initially aquacultured for food production or aquarium trade (i.e., initially not conservation dedicated) there are several additions such as various sturgeon, various large Asian barbs and catfish, Chinese high-fin banded shark, arapaima, Asian arowana, dwarf loach, white cloud mountain minnow, flame tetra, diamond tetra, etc. Aquatic but not a fish; it seems the Chinese giant salamander, now farmed in large numbers, may follow the same path. The future for the wild population seems increasingly bleak.