how many guppies should i put on a 20g tank considering its adult size and how big do guppies become? i also have a problem differentiating a male and female guppy. i need your help guys. thanks and more power.

guppies are breeding very very fast ... so you could add 10 guppies and in 2-3 months you will have triple.

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Male guppies are the smaller ones with the bigger flashier tails. Females are bigger and have smaller tails. I have some orange sunburst guppies and i had 2 of them for about 3 weeks now i have 14 fry in another tank. Im just going to give them to a good local fish store when they are grown up a little. I thought guppies would be great for my girlfriend now we just end up having to deal with fry all the time. If you want small fish for a 20 gallon tank i would go with neon tetras and cherry barbs, they wont breed as much. Unless you wanna deal with live births every 4-5 weeks i woudld stay away from guppies, especialyl since they are usually sold in a breeding pair.

Although you can normally tell the difference between males and females that way, as the males are the smaller and flashier ones, the definitive way to sex them is to see if they have a gonopodium(male sex organ) or an anal fin if female.

A gonopodium by the way is a longer pointy looking fin that hangs down. Only males have them.

You could get only male guppies and then there would be no babies.

how about the molly fish..
how many can i stack in a 15gal tank?
i dont want them to have plenty babies btw.

I would get no more than 8-10 Mollies in a tank that size. A mature Molly can get in the 2-2 1/2 inch range. Same rule applies to Mollies as all livebearer fish. If you mix them, they will mate. Mollies, given their size, can have as many as 30+ fry at one time. I just had one that had 25. So stick with all males.