Tips for keeping guppies alive?

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I've always struggled with keeping guppies alive. Idk why but I usually lose at least 1-3 during the first night. I just bought 10 of them from a petsmart since they were cheaper than my LFS but I'm gonna be real they were on the skinnier side compared to my lfs selection but I just ignored it. My tank had been cycling for around 2 1/2 months with filter media from a previous tank to boost the cycle and my ammonia and nitrites were 0 before adding the fish in. I'm not really sure what I'm doing wrong since I've kept tons of different kinds of fish in the past and the only ones I have a hard time keeping are guppies.

 

[ember04]

it could be a few things
Unfortunately, because guppies are super cheap often shops don't put that much effort into them so they come in underweight as you said, it can be hard to get them eating again and often they might be too far gone. It is always better to spend those extra few pounds or dollars or other currency to get a healthy well-fed fish.
Sometimes guppies can just be weak, over teh last few decades they have become really in and over bread and it resulted in less hardy weaker fish. Not all of them but some of teh more fancy ones.
Sometimes these fish as they are cheap don't get medicated which means these fish may have parasites.
Also guppies tend to thrive in harder water so depending on your water parameter they may not be able to thrive
Hope that helps
Ember