To increase growth rate you need:

1. more frequent and bigger water changes
2. more live food, with emphasis on Baby Brine Shrimp, variety and frequency
3. a bigger tank if possible

Growth development also depends on the fish's genes, but at 2 month, they definitively be larger. My 1 month old are ussually 1 in long.

I used 30 gal tanks for raising Guppy fry. I feed several times a day and cultivate my own Brine shrimp. I feed them earthworm flakes, black pearl flakes, color flakes, multi flakes, spirulina flakes, anti-bacterial flakes, anti-protozoa flakes, bloodworms, Daphnia, Infusoria and everything else I find available including green water sometimes.

The more BBS and live food with Carotene they eat, the brighter and stronger color the fry will have when they mature. So, try more WC and changing the diet. That can already make a difference.

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GUPPY STRAINS I'M BREEDING:

Blue Grass
Blue Grass Swallow
Metal Blue Grass Swallow
Red Grass
Red Grass Swallow
Neon Blue Giessen/Ribbon
Japan Blue Blue Grass
Japan Blue Wild
Neon Blue

Strains I developed in my fish room:

HB Yellow (own line)
Platinum Pink Blue
HB Blue Leopard
HB Leopard Multi
HB Red Crowntail