I'm sorry to hear you have lost your fish :(
The white fuzz you saw was most likely the slime coat sloughing off when they die- he may have been gone longer then the others. Fungus you tend to see it progress and it takes a bit to actually kill the fish.
Some of them won't eat right away after getting them home- sometimes some will take a week or so before they eat healthy.
What size bowl is it? Is there a heater? Did you use water conditioners? Did you use anything to clean the bowl/rocks with? Did you acclimate to both temp and chemistry of the water? Which is important when you first get the fish to acclimate to the chemistry as most likely their cup is full of ammonia/nitrates.
Bettas are tropical fish, and like other tropical fish you see in aquariums, they require a certain degree of temperature. 76-82* range is what they are needing- without fluctuation. Leaving them in the window you will subject them to drafts, as well as fluctuations between night/day which could cause them to go into shock. Being too cold will also cause lethargy, slow metabolism (which in turn means less food consumption) and lowered immune system.
One plant may not be enough for cover, why they may not be exploring or swimming around- not a whole lot of mental stimulation or places to hide to feel secure in the new home. They like a fairly well planted tank (silk or real plants) and most love and sleep in caves.
Bettas are hardy and can withstand unfavorable conditions for a while- but with the mass breeding going on with them their immune systems and genes aren't always all that great- so sometimes you have to take extra care to make sure they are acclimated, the temp is correct, etc when you first get them as to not put them into shock.
When you get another, make sure to wash the bowl with super hot water, and decorations.. fill it up with the correct temp, add in water conditioner.. float the cup for about 20 minutes, 3-4 times within that 20 minutes remove about 25% of the water from the cup (dump it) and replace it with water from the bowl. When it's time to release him- using a net, gently pour him into the net from the cup and place him gently into the bowl. Don't let the cup water into the bowl.
Place the bowl in a warm part of the house- not near a window/door/air vent if there is no cover on it- if there is a cover then you can get away with having it near one of those- just as long as there is a heater in the bowl to keep him warm and steady temp.
For an unfiltered tank 1-4 gallons weekly 1 50% and 1 100% water change is needed (with conditioner)- for a filtered 1-4 gallons weekly 2 50% water changes- one being water only, the other being substrate cleaning (with conditioner).
Offer food daily (should be fed minimum 2x a day- 2-3 pellets per meal), remove any uneaten right away so it will not foul up the water- don't be discouraged if it takes him a while to eat, he will when he is adjusted and feels secure.
I wish you luck- sometimes we do just get an ill fish when we pick them out, and we can't help them...
Last edited by Myates; 01-03-2012 at 12:37 PM.