I basicly asked the same question about my tiger fry. I was told fish will eat anything they can fit into their mouth( including other fish). Try different foods, smaller portions, more times a day. I use frozen worms and shrimp. If they are to big for the fry to eat grate them with a cheese grater add tank water till defrosted and let them have it. Research your fish and find out their diet.

 

Slow growth can be attributed to a few things, one is space as you are pointing out, two is diet like j dizzel pointed out and three is water quality, all of these things need to be accounted for when raising fry, I actually have some miniature fish now due to less than stellar water quality when they were fry. Unless these things are taken care of from day one then the damage probably has already been done although they might get bigger as they age, they will never get as big as they should.

At 2 cm I would go ahead and release them with the adults, should be just big enough for them not to bother them too much, even if they do chase them around some I doubt they will harm the fry.

 

As Zof said, my main concern would be water quality. Nitrates will severly limit the growth of fish, it's natures way of controlling population-delayed maturity means fewer new fish will be born. This is the main reason why breeders usually raise fry seperately and do lots of water changes.

If they were growing well before, I would say diet is not the issue (unless you've recently changed your food supply). Another thing with that-juvenile fish have HUGE appetites, and at this point they should basically be growing in leaps and bounds and eating to match.

Your fry are probably ready to be released, once they've had a chance to develop good muscle tone, they usually have no trouble evading their parents. Since you only have three, I can understand you wanting to make sure they all live, but your adult livebearers will be producing fry every month or so and if you save them all, you will crash your tank.

Simply, I would do a water change and see if that doesn't help.

It's a myth (albeit a popular one) that fish will only grow to the size of their environment. If that were true, you would never see an eight inch pleco in a twelve inch aquarium. You wouldn't even see a six inch plec in a ten gallon as that is still too large for the space. But sadly for the common plecs, this happens all the time. Size is mostly determined by water quality and diet. Fish that fail to reach their normal adult size (such as common goldfish) are usually fish that have been subjected to horrible conditions and are thus stunted, rather than some inherent ability to self-limit natural size.