How often do you feed them?
How much do you feed them?
Their diet could do with a bit more variation.
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To grow fish quickly, you want warm water (28C), lots of big regular water changes and gravel cleaning, a clean filter, and a varied diet fed regularly. Miss out on any one of these and the fish won't grow as quickly as they could.
The other reasons fish don't grow is parasites like gill flukes and intestinal worms, and bullying from bigger or more dominant fish.
The fact some fish have grown but others haven't would suggest bullying, lack of food and parasites like intestinal worms. Naughts mentioned deworming the fish and that would be a good way to start.
Feed the fish 3-5 times a day and give them as much food as they can eat. Don't put it all in at the same time, just offer a bit and let them eat it, then offer a bit more. Continue doing this until they are full and stop racing in to eat the food. You should add raw/ cooked prawn/ shrimp, and other marine foods (fish, squid, etc) to their diet. You can buy frozen foods like brineshrimp, mysis shrimp, daphnia and marine mix (prawn, fish & squid blended up) from most pet shops and these can be fed each day in addition to the dry foods you use. You can add some plant matter, especially for the rainbowfish, whose diet should consist of 50% plant based foods.
Do big regular water changes every day or two when feeding more often so the tank remains clean. Maintain the filter too.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Deworm all your fish. See section 3 of the following link to deworm the fish for thread and tape worm.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
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If there's no improvement after that, then it is probably bullying or the fish are genetically stunted.