Betta died

Question
Hi,
someone left our company and she left a Betta fish in my care.  He was in a small bowl but seemed to be ok even though her feeding was irregular and so was the cleaning routine.  When I got the fish, I bought a new aquarium, heater, proper rocks, plants and lots of testing stuff, etc.  Fish seemed to be fine for a while but after one of my water changes, he did not do well.  He died a few days after.  Now, I am afraid to get another Betta because it's clear to me that something I did killed the poor thing. He was a "prize" at some party she went to and he was not very colorful and his fins were a little ragged.  He must have been 2 or so.  If I changed the water too often, despite adding the stuff that takes out the chlorine, could that have killed him?  How long should I have the water sitting there until I can add a fish?  
Thanks in advance!!

Answer
Ragged fins could be tail rot. If it was, it was the previous owner's unsanitary conditions that killed the fish, not you. The only thing you may have done was stress the little guy out by changing the water too often. But 2 is old so I wouldn't feel too bad. It wasn't your fault. Clean everything really well and start over with the tank and get a new betta (and a betta care book for the owner)