In my opinion, water changes (to dilute the growth hormone) or rather the water volume has the biggest impact on growth.
There will always be a size variance in the bettas among a spawn.

The amount of dilution of the hormone in the water will inhibit the growth of other bettas (other competitors) and also inhibit the growth of the betta itself to prevent itself from overgrowing it's own "territory" (tank)

Food does play an important factor as well, but is secondary, after the initial growth phase when they are still feeding on baby brine shrimp and infusoria, food quality and food amount plays less of a role because of the growth inhibitors of the growth hormones released by the fry.
(for example 10 fish in a 1 gallon fed a high quality food will grow much more slowly than 10 fish in a 20 gallon fed poor quality food.)

I once split a spawn at around 1 month old (no giant/king genetics), half (around 10-15) into a 500 gallon pond, and the rest (10-15) in a 20 gallon tank, at the end of the season I went to collect some of the specimens from the pond, and in general the pond bettas were much larger at nearly 2 inches body length, while the tank grown were around 1.5 inches body length on average. I understand there are a lot of variables that could've affected them such as temperature and food, but even when providing tank raised bettas with high temperatures 82-86 degrees and 3 generous feedings of live/frozen and occasionally pellets 3 times a day, they never grew as large as the bettas that grew up in the pond.

 

If you did all the right things as discribe above, it could also be another cause, we all as breeders had to cope with this from about 2018 and some months before.

no spawn were lucky and also some fish didn't spawn at all.
We did all the same as we use to do in the grow out tank, live food, waterchanges daily, keep the ground clean but no growth and all stay a small size like 2/5 cm.
Egg eating and even fry eating, wich for me is quiet rare.

All the fish were in excellent condition, but they all stayed behind, so some spawns we had to cull because they didn't grow and than they were over a year.
We all had this discussion on fb and together, causes weren't find but I do think that ones in a while or year the nature is completly, like I tend to say, out of the road.
So now we have some spawns going on again and growing nice. And still exactly the same as we always do. So the staying small could also have another cause than only lack of good food and waterchanges.

I do not know if more breeders are familiair with this fenominum but here in the NL we all had this cause going on, even the smallest and biggest breeders [emoji21] can have to cope with this at home.

So after this story, that could also be a cause of no growing. [emoji6]

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I personally think it is likely poor nutrition that has caused the stunting. I say this because when I was breeding wild bettas, I would perform very few water changes. Perhaps removing and replacing 25% of water once every month or so. However, this did not prevent most fry from attaining their full adult size. Instead, it seemed that the fry that were less aggressive eaters were the fish that grew much more slowly. The difference in size only increased as the fry grew older, and even now, there are fish that are only about half the size of their siblings, even though they are two or even three years of age.

I've seen the same thing occur even with very frequent water changes. There becomes a very clear difference in the growth rate between those fish that are first to feed and that eat well, and those that do not.

What else are the fish eating now apart from frozen bloodworms and frozen brine shrimp? If you want to see faster growth rates, you need to introduce more nutritious foods, such as blackworms, white worms, or grindal worms. Even a high quality pellet or gel food such as Repashy would do. While some fish can be permanently stunted, you may see results with a better diet. This is of course in conjunction with frequent water changes.

 

Along with the frozen Brine shrimp and frozen blood worms, I also feed them freeze dried blood worms, Tetracolor Freeze dried krill (they love this, but I have to smash it into smaller pieces for them), flakes and granules.

Also, during the summer I had some outside water buckets that I used for my chickens water refills (it was fresh water I stored in a covered shed for later use) that was filled with mosquito larva. I would feed the larva to the Bettas when they were growing.

Some of my Bettas I still have from that spawn is housed with some Japanese Trapdoor Snails, so they get fed some homemade snail food, and the Bettas sometimes nibble on it. I made the gel food with crushed calcium tablets, some high protein baby food, shrimp pellet formula and gelatin.

With these foods, I mix it up a bit on which food they get on what day.

About the Repashy, which gel mix should I look at? I am trying to avoid live feed right now because we (my family and I) going to be moving soon, and I would hate any of my live cultures to die in the process of transferring.

 

Best to research BEFORE you do it