If your rainbowfish are taking 6 months to reach a saleable size, they are growing slowly. Most Melanotaenia species should reach 2 inches in 3 months.
My fry were grown up in 2 foot tanks.
The water temperature was 28C.
The room light was on 16 hours a day.
There was a thin layer of gravel on the bottom.
The tank contained an air operated sponge filter that bubbled slowly while the fry were in the plankton stage (clear and hang out under the surface), and I increased the aeration (bubbles) when they became silversides and moved down into the main water column.
The tanks had coverglass to stop evaporation, reduce heat loss, and stop fish jumping out.
FEEDING
The fish were fed 3-5 times a day using a variety of foods. They got infusoria and powdered fry food for egg layers during the first few weeks. Newly hatched brineshrimp were added at the end of week one. When all the fry were eating the brineshrimp I stopped adding infusoria. The fry then got powdered fry food, microworms, and newly hatched brineshrimp were added last. I added enough brineshrimp so there was plenty floating around the water and the fish would eat them over the next few hours.
When the fry were silversides, they still got fed 3-5 times a day and got dry food first, followed by finely cut up marine mix (prawn, fish & squid blended up). Then frozen brineshrimp, bloodworms or daphnia. Then microworms or grindal worms (depending on the size of the fry). After they had eaten that, I added a heap of newly hatched brineshrimp so they could eat them over the next few hours.
The fry should be fed as much as they can eat and look like pregnant guppies at all times.
WATER CHANGES
The tanks didn't get any water changes during the first week. After that they got a 10-20% water change using water from the parent's tank. When the fry were all eating newly hatched brineshrimp, I started doing bigger water changes using aged dechlorinated water that had the same chemistry (pH, GH & KH) and temperature as the rearing tanks. When the fry were around 3-4 weeks old, they got a 75-80% water change and gravel clean every day.
SUMMING UP
- Keep the water warm (28C is the best temperature for rearing tropical fish fry).
- Feed them until they are full and add some live newly hatched brineshrimp after that so they can keep feeding for a few more hours.
- Do lots of big water changes to keep the tank clean and dilute any growth slowing hormones released by the fry.