New Tank - Fish Dying

BettaDexter

Bad news :(
I did a 5 gallon swap of the water yesterday immediately after posting to the board. After I finished working I went to the petstore I purchased the tank from and checked the levels - everything was looking good. Everyone was acting ok. I didn't pull the betta immediately since everything seemed fine/he was acting normal to not shock his system.
All of a sudden last night I notice around 9pm he's hiding down in a corner of a piece of driftwood on the ground. I watched him until about midnight, wouldn't move for anything. As a last-ditch effort I pulled him into a 1-gallon of half existing tank water and half new(treated) water. He's literally dying right now - stopped breathing through gills, takes a breath from the air, starts to tilt, repeat. I don't know if the water change was too much for him, maybe I should have taken him out before. He's had his 1-gallon fully changed all the time with no issues.
I tested the water again last night with my new kit, everything was under control, the pH was HIGH though - 7.0 when they tested at the store, 7.8 when tested at home.
What's most frustrating is he's the reason I upgraded the tank in the first place/I feel like crap for killing my betta.
One thing i did notice - when I removed the dead fish yesterday it had its fins nipped up. Was definitely done after death, the others are unhurt and noone is nipping. I wonder if it wasn't an ammonia issue at all and he was just ill/the betta chewed on him and caught whatever he had.

BobtheSnail

"One thing i did notice - when I removed the dead fish yesterday it had its fins nipped up. Was definitely done after death, the others are unhurt and noone is nipping. I wonder if it wasn't an ammonia issue at all and he was just ill/the betta chewed on him and caught whatever he had."

Wow! Good eyes! It's possible that he had something. Did you notice anything odd about the fish before he died (the one the betta supposedly nibbled on)? Any odd coloration or fading, swelling, loss of appetite, etc.?

BettaDexter

Not that I noticed - but they again I'd only had the tetras for 2 full days so I wasn't really sure of their usual behavior at that point.
Not sure what I'll try next - I know I need to give the tetras some more to form groups with their own types, a small pleco and a couple ghost shrimp. I'd like an angelfish or a one a bit larger than the others but I don't think my tank is big enough(15 gallons, about 18" wide by 24" high or so) Not in a rush, will see what happens

A quick google search later and the Angelfish is clearly a no-go. Tetra tank for now.