Nothing in my fish tank is surviving and I don't know why, help?

I got a 5 gallon fish tank, cycled it, conditioned it, it's heated to 75 degrees, everything in the tank is cleaned including the gravel. I decided to keep a betta fish in it. I got him from Walmart and he didn't look so good to begin with so I assumed he died in less than 12 hours because I got him in poor... show more I got a 5 gallon fish tank, cycled it, conditioned it, it's heated to 75 degrees, everything in the tank is cleaned including the gravel. I decided to keep a betta fish in it. I got him from Walmart and he didn't look so good to begin with so I assumed he died in less than 12 hours because I got him in poor condition.

Tank has a 50 percent water change. Fast forward a couple days. I decide to get another Betta fish, 4 black neon tetras and 2 ghost shrimp. Within the first 6 hours 3 of my tetras are belly up. I believe the first 2 got eaten because I can't find their remains. The third I scooped out of the tank and the 4th seems to be doing fine. My ghost shrimp are thriving and active as ever. The betta is hiding behind the filter and not very active which has me worried. He was extremely active when he first entered the tank but now he's very sluggish and not very mobile. He will peak out occasionally but just sits near the top of the tank.

Does anyone have any idea as to why my betta are not thriving like my shrimp? I'm very confused and I can't find any good advice online for what I am doing wrong. Thank you I appreciate the help!
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