Please help save my fish Stefan!!!!

Ok I have had my pet Betta fish (Stefan) for 3 days now and when we bought him we also bought another betta but the next morning it died but I still had Stefan and i was so upset that my mom bought me 2 goldfishes but in 3 hours they both died so i got on the internet and found out that tap water kills them so since i still had Stefan i got him a 3 gallon tank and water conditioner a heater becasue they are tropical animals and a filter with some little decorations but arent betta fish supposed to flare there fins or gills or whatever but he hasent did that and i don't no whats wrong and there is this black spot on him can someone tell me what it is???or whats wrong???PLEASE HELP ME!!!!!!!!!!!


Here's the black spot:link

and here is a pic of him he always swims like this he never flares his Fins/gills! :link