The bigger tank the better. 40-50 gallons is perfect for assorted guppies, 10 gallons are fine for 1 male with 5-6 females. Also 10 gallons for female birthing tanks and fry grow out tanks---
As for saving fry,
Sometimes you get good results if you put the [pregnant/gravid] female guppy in her own 10 gallon tank. Add gravel, decorations, fake and real plants {floating and none floating}. Add pond snails to the tank to clean up excess food. Feed the gravid guppy 3 times a day,
feed a varied diet, bloodworms, algae wafers , flakes, shrimp pellets ect.
Stay on top of water quality. Use water from the tank the fish come from when moving fish from tank to tank. {[if the water is exsactly the same the fish will not get stressed as much!]} {in other words, fill her birthing tank/ with water from the tank she just came from!} The birthing tank can be a simple 10 gallon with no filters or pumps. As long as there are plenty of surfaces ( gravel, plants,decor' ) for bacteria to live on , one guppy should be fine. Always monitor water quality unless you fully understand the nitrogen cycle! No matter what you do the mother guppy can eat all of her fry! Shes the boss. Watch her close, take the fry out as soon as possible---
You should have 1 male to a tank with 5 or 6 females, If you want them to breed true , they should be together for 5-6 months, then the fry will be from these fish. Do not put several males in one tank unless they are the same color.{ or if you do not care about colors and genetics. } Good luck!
