Preventing Betta death

Question
I'm from Queensland Australia where it is warm. As I'm new to keeping fish, the Pet Shop assistant advised me what to do. I have followed his advise but something's not right and hope you could help me understand better. I have recently bought 2 Bettas about 6 1/2 weeks ago. I put them in a tank with a divider. They seemed happy at first but one of them started becoming listless and playing with his food which he eventually ate. I did a water change on the 4th week and this morning I saw he was pale, didn't eat, was tilting on his side and was relatively still. I had noticed a few weeks ago that his bottom fin was stuck together somewhat knotted and it wouldn't spread out. I did a water change and added salt and Stress Coat. The fish kept on going to the top of the water to get some air and he would occasionally spin around real fast. We kept an eye on him and after a couple of hours he sunk to the bottom laying on his side and died. I then did another water change. I was not advised to do a 100% change. Do I need to change the water 100%? Meanwhile the other fish seems fine but we're keeping a close watch on him. Will he be contaminated too? Also what had caused the fish to die? I was told by the shop assistant that I didn't need to check the ammonia or pH, is that true? We have been pretty upset and very helpless and wish to prevent the other fish going through the same ordeal.
Thank you for your help.
Regards
Susanna.

Answer
Hi Susanna;

Poor guy. He had finrot and probably internal infection. The fish store may have misinformed you about betta care and maintenance. I'm so sorry this has happened to you and your fish. It's all to common however. So many stores are giving out the wrong info or just none at all. It's a shame.

In tanks with no filter, bettas need to have a 100% water change weekly. If the unfiltered tank is less than a gallon in capacity it needs a 100% change twice a week. If the tank has a filter, you can do weekly changes of 25% to 50%. In any case, remove leftover food right away so it doesn't pollute the water. With a good maintenance schedule like that you really don't need to worry about what the pH, ammonia or nitrite levels are.

Here are some good links to web sites about betta care and their ailments;

http://www.bettatalk.com/
http://www.healthybetta.com
http://www.mushi.petfish.net/bettapage.html

Your surviving fish will probably be okay as long as you maintain his tank well from now on.