i feel for u freshwater. when i first started, i had alot of problems to. my first fish i was able to keep alive was zebra danios. they are so hardy, that when i did my first water change i forgot to put in dechlor, and they all survived. they fishy cycled my tank for me, and are great fish for a beginner, and are very active.
now, i was thinking about a typical water change, and thought of everything thing that could go wrong. first, how close was the new water temperature compared to the tank temp. a few degrees is acceptable, but more than 5 or 6 degrees could have done u in. and the other question about dechlor. do u know if your water contains chloramine, as some dechlorinaters do not remove it. i suggest, stress coat. thats wat i used to use on my freshwater tanks, and what i use on my saltwater tank. it removes metals, chlorine, and chloramine, and has aloe vera for the fishes slime coats.
as for the betta, do whatever you feel is right, its your call.
sorry to hear you are having a tought time to start out, and i had a hard time to. just stick with it, it will pay off when you have your dream tank, and do a lot of research. don't just listen to the people here, they give there opinions and personal expierences, and that may or may not be right for you in your particular situation. but there is alot of good info here. tank wat u read here, wat u read other places, and your judgement and expierence and you will be sucessful. this hobby is a hobby of trial and error, and once you get by the iniatial phase, it is very rewarding. stick with it and good luck :thm: