Self regulating breeding groups?

[GaryE]

In another thread, the question of self regulating breeding groups came up - what happens when you don't move out young fish hatched or born in your tanks?

I don't keep lists, but I have bred over 200 distinct species of fish at this point, and I've seen some things as a result. This may upset the rule making people out there, but it's an interesting question because a lot of conflicting answers are correct. And my experience is going to reflect my aquariums - the size and set up of different tanks can produce different results.

I have some fish that maintain steady breeding populations over many years. I have some Chromaphyosemion poliaki killifish that have been in a 40 gallon for 4 years at this house, after thriving in similar smaller set ups at the old place for 15 years. The population is always about the same size, but I occasionally remove pairs or older males, and share them out in the local club. They're pretty, so there's a slow steady demand. If I leave this 40 gallon jungle to its own devices, I get older males picking off both young males and females, as the tank is a touch small. I think in a six foot tank, it would be fine.

I recently lost a species in a similar set up because I ended up with only males. If I had been a bit less trusting of the process, I would have managed it better. The problem wasn't the fish, but the aquarium's limitations.

I've also had some species build and build til I had a crash - a disease would sweep through and regulate the population, often by eliminating it. That's happened with fish that breed faster than people want them.

With Cichlids, often the easiest breeders are aggressive, and such tanks end badly. I used to keep Neolamprologus marenguensis, a beautiful Tanganyikan that effectively built villages. Each brood aged into roles and contributed to the collective good. They'd dig, maintain nests, care for younger siblings, defend the perimeters along with their parents... it was one of the coolest fish spectacles I've seen. In the wild, they and related species have had 'villages' that divers have tentatively estimated at 20,000 individuals. I had to cap it at a hundred or so and that tank became a lot of work to maintain water quality in. I had five to seven broods all working together - great fish. They were a far cry from convict Cichlids ripping their siblings eyes out.

In between, I had Pelvicachromis kribensis Moliwe in a 55, and from broods of 50, 3 or 4 young grew to breeding age and took territories in with the parents. But when they spawned, small tank size ended that show.

For me, every apparent self regulating population tank has crashed within a few generations, if I didn't act like a predator and remove individuals to send them off to friends and fellow aquarists. What has been the experience of others?

 

[Gertrudae]

Three species have been living here for decades without regular intervention.
Endler's livebearers – since around 1990. Initially in smaller tanks, since around 2002 in 325 litres until 2024. I may have given away or added some fish three or four times.
I still have this strain, but with more interventions, as some fish lived outdoors in the summer and then came back. And they are now in an aquarium that is only half the size.

Tanichthys albonubes for about 20 years. They lived in various aquariums, mostly 200 litres. Once I added 5 new fish. I usually saw a few young fish grow up in the spring, rarely more than 2 to 5 at a time. Three years ago, they moved into my new 325-litre stream tank.
They spent last summer in my new pond in the garden and came back in incredibly good condition and with bright colors. At the moment, there are more than a dozen young fish that are almost 1 cm long and have this beautiful neon blue stripe, as well as several dozen smaller young fish. I am curious to see how things will continue and whether some of them have survived in the pond. I'm sure I didn't catch them all in the autumn.

Danio margaritatus is almost the same story as Tanichthys, except for the last part. They still live in 200 litres and haven't spent any time outside, but they will this summer.

I think the size of the tank is important. Inbreeding ist not a thing in fish. There are many species that were introduced once and/or with a few specimen and they thrive since decades in the hobby.

 

[Innesfan]

I've only ever done this with Nannostomus, corydoras and killies. It worked less well with larger killies, i.e. the bigger Fundulopanchax, which is to say overpopulation and skewed gender ratios became an issue unless I vigorously and frequently thinned out the group. I had a crash or two in killies. Never with Nannostomus... so far.

 

[TwoTankAmin]

The answer depends first on the species. Next on the tank size and set-up and finally on the inclusion of what tankmates or the lack there-of.

I bred mostly plecos and if I did not remove offspring over time the tank got overcrowded and crashed. I always did/do intentional breed attempts in in dedicated species tanks.