I just "matured" 33 to the 7 week stage in 2 each 2 gallon aquariums. WIll not do that again, but here was my technique:
I started with just one tank but with about a 95 % water change twice a day. I was feeding really healthy quantities of brine shrimp. After about three weeks I could discern the gender and split males and females, each into a 2 gallon tank. Each was aerated with a stone and heated as well. I kept up the twice daily water changes. I never knew how much a guppy could poop! Moved them to "FIrst Bites" after two weeks, and then eventually to "Micro Pellets" and , believe it or not, " Omega One goldfish flakes, ground up fine.
I didn't loose one fish and I believe that that were well treated. Their water never became nasty ( I tested for ammonia daily) because of the twice daily changes and I felt that there was more than enogh room for them to move, swim , and be happy according to their size.
Just this evening I spread them out amongst a couple of "grow out tanks", up to 10 gallons, dividing out the one I will keep for breeders , and I think I have done well by my fish
1. They were never in dirty water.
2.. They were always well oxygentated.
3. They always had enough space and exercise room for their size.
OBTW, the tanks were bare.
Jack