guppies dying overnight

i am new to the hobby and i’m not sure if i’m doing something wrong or if it’s some coincidence so please let me know what you guys think.

i have a 10 gallon planted tank that i was attempting to keep guppies and a betta in, however i ended up putting the guppies in a different tank after finding 2 of the females dead a couple days later. so i moved them into another 10 gallon tank that i didnt cycle (i had never heard of cycling until recently) BUT i used water from the 3 gallon thank i temporarily kept them in until i got the bigger tank. i checked the water perimeters in it before putting it into the 10 gallon and again after i finished setting the tank up. i also used the same filter from my betta tank on this one. i ended up buying 4 more female guppies because i only had 1 female and 2 males left and the males were bothering the female. the first 5 guppies (2 male 3 female) i got from a smaller pet store and the second batch of girls i got from petsmart. i came home today to find 1 of the boys and 1 of the girls dead, which were both from the first pet store. the girl however, i noticed yesterday had a crooked spine so that may have had something to do with it.. but that makes 4/5 of the OG guppies that have died in a matter of maybe a week or so basically over night. someone please help me because if i am doing something wrong i would love to know and change it!