IPhone charger is literally de-charging my phone?

I just bought a new iPhone charger (not an Apple product. First mistake) and it's literally taking battery life away lol. I plugged my phone in at 12% and it's going down…
I can return it, not a problem.
I'm just wondering if this can be fixed. If not, anyone have a clue as to why that would be? I'm curious now lol

Added (1). It's a wall-charger. It charges fine with my computer. Very strange! I've even tried switching out the USB chord for another one to see if that was the problem--it's not.
So I guess the phone's battery is fine?

I've never heard of this and I wouldn't worry about it. Just get a refund and be done with it. Next time, but official and it'll be all good.

How are you even charging your phone? Does any other chargers work? Could be that the battery is fried. I've read that you never want your battery to "die" when it gets down to 0%, it takes "life" away, and supposedly it will little by little make it so it doesn't hold a charge as long. You want to start charging it probably at like 15%.