Beta fish dying

Question
Hi,

I have had two male beta fish in a 1 gallon tank with a divider for some time.  I also have a whisper filter  and some figuerines such as a diver and treasure chest in the tank.  The tank comes equipped with a light with a 15 watt bulb.  I change the water regularly and have had no problems after changing the water with the fish, however I have just recently changed the water used beta safe solution and some algicide " safe " Solution as well, and put the fish back in the tank.  They were dead in less than 4 hours.  My son was upset so we went and bought 2 more beta's from Petco, and I cleaned out the tank put the beta safe solution waited several hours and finally transferred the fish to the tank.  These fish were found dead this morning.  A factor that might  be an issue and am not sure about is that the bulb of this tank burnt out, and I went to Petco to buy another 15 watt long bulb.  Petco didn't have the same bulb, but had one of those new type of bulbs that are considered  energy efficient 1/2 white and 1/2 blue tint 10 watts.  I put this bulb in after I changed the water with our original fish. Could the bulb be the cause of the fish death? The light wasn't on all the time, but was put on for the first couple of hours so that we could see the fish.  
I am concerned because I don't want to put any more fish in this tank if they are just going to die.  Please help.

Sincerely,
Judy  

Answer
First, it is never a good idea to keep two male Bettas in or near each other.  They get stressed because they feel the need to defend their territory.  Secondly, is there a chance the water was too cold?  The older bulb could have been heating the water for you.  Betta's like to have warmer temps of about 78-80F.