yep i know how that goes i had a male betta in my big tank and he had pop eye and i couldent stand to watch him suffer.so i put him down too,bettas never really do very well in big tanks.
i do feel kinda bad for putting down the last two fish for basicly no reason but i gave them a good life and plus i didnt plan on buying anymore tetras and they are schooling fish so that guy was pretty sad all byhimslef so i sent him to fishy heaven with the rest of his tank mates