With guppys you should have at least 2-3females per male as the males will often harrass the females too much, and by having many females you avoid the males over-harrassing or raping the females- guppys have extremely hig sex drives are the rabbits of the fish world, be prepared for fry as they are also known as "millions fish"

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As noted before, stress can show itself in many forms- theres many things that can stress fish out, some of the more common things that stress fish out are;
a. Overstocking your tank.
b. Water quality issues in the tank.
c. Big Temperature changes or ph fluctuations.
d. Mixing fish shouldn't be mixed together, or not addressing a species of fish's needs i.e keeping it solitary or in a school, not feeding it right or adressing its living requirements like giving it a cave to rest in or fine substrate to feed off etc etc..
e. Mixing the worng meds or over-using meds, or even adding meds that particular fish can't tolerate.
f. Deseases.
etc etc...
The main problem with stress and fish is that when fish are stressed, their imune systems don't work as efficiently, and they are more prone to picking up deseases and being overwhelmed with them. Fish deseases will spread an aweful lot faster in and overstocked tank, and will be harder to treat if your fish are stressed for other reasons than just being ill.
Many fish will just die of stress alonem or will encounter over issues with stress, for example, with pregnant female guppys, they are far more likely to suffer birth complications if they are very stressed.
I could go on but you get the jist of it- being a successful fish keeper is all about aiming to have chilled out non-stressed fish- if you have a non-stressed out fish you have a happy fish

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