My Betta died... : ( Why?

I've been keeping two male bettas sans filters. Got them last autumn. One has been doing good and regularly builds nests. The other one came down with fin rot during the winter. Recovered from the finrot, except now his fins are as short as a fancy female betta.
 
Bettas can do fine in tanks as small as .5 gallons with daily (and i mean EVERY day water changes). In fact, they can even prosper in smaller quarters (thai breeders often raise their stock in 250 ml whiskey flasks), but it significantly increases the amount of work that you have to do to keep them healthy. If you are doing weekly water changes, you definitely need a filter. If you don't want a filter, start with the daily changes. Either way, your next betta will be much happier if you arm yourself with as much research as you can.
 
I would reccomend something more than one gallon with a filter. In the colder months a heater. Regular waterchanges are important. bettas are sensitive like all fish to high levels of amonia and other things. I breed bettas and I have bettas in 1.5 gallon tanks 2.5 gallon tanks and 10 gallon sorority. gravel vaccing is also necessary. and if you knotice lots of extra food in the tank cut down on it. What are you feeding your bettas?
 
AquariaCentral.com