Before I tell my story, I'll give some info about my tank:
The tank in question is a 10gal with 6 zebra danios, 2 spotted cories, a pleco, and 2 apple snails. It is heated and filtered and has a bubbler. I do 50% water changes with gravel vac whenever I think it needs it (like once a week) and 100% changes when the substrate gets substantially dirty. I use a double dose of Stress Coat + as my water conditioner.
Okay, now my story:
A few days ago, I brought home a new betta from Petco. I acclimated him and put him in the 10gal and everything seemed just fine. He was active, eating, and swimming around just fine. The next day, he's lying on the bottom of the tank breathing heavily. I took him out and put him back in his cup to make it easier for him to get to the surface. Within an hour, he was dead. Since he was new and from Petco, I wasn't really surprised, so I thought nothing of it.
The next day, I took my betta that I had in a 1gal and put him in the 10gal, thinking he'd like the space. I've had this betta in the 1gal for months and he has been perfectly healthy. He goes in the 10gal and everything is fine. He's active, exploring, eating, and acting perfectly normal. A day or two later, he's lying on his side on the bottom of the tank breathing heavily. Again, I put him in a cup and he's soon dead. This was a betta that was perfectly healthy for months. Also, something I find interesting is that during all of this, none of the other fish in the 10gal have been affected in any way. They are all still active, playing with each other, and eating just fine. I'm so confused..