Guppy In An Outdoor Tank

[GriffinC18]

So i placed my female guppy that i believed was pregnant in a 15 gallon tank outside. The temperature was perfect, day and night both....i constantly checked it.  It ended up that the guppy wasn't pregnant even though she looked like an Elephant in the tank compared to the other female. I moved her back into an indoor tank cause i was tired of removing all the debris and such that kept falling into the tank....plus the water was turning greenish/yellowish. The tank was filtered.
 
 
So my question is, what do you guys suggest about outdoor tanks?
 
Also, i only fed that guppy flakes twice in the time that she was in the tank.............She looks healthy and still bigger than ever, i thought that she would eat the stuff that fell into the tank but i do not know whether she did or not.
 
When i recently fed her, she ate about two flakes like she was starved...then wasn't interested at all like she had just ate a seven course meal.

 

[Alasse]

In warmer months i often have guppies (and other fish) outside

 

[DreamertK]

I would half worry about algea due to all the sunlight (unless you had otos), which would explain the green water you had.