Bad mix of fish and too many too fast. There is no way you can have that many fish in so small a tank. Has anyone told you about cycling a tank?
A tank needs good bacteria to eat the waste the fishes produce. When you set up the tank with its under gravel filter (ugf) it was ready for the bacteria to grow but they can't do that until you add some fish to produce the waste. More waste = more bacteria and vice versa. As the bacteria grow you can slowly add more fish and so the tank 'cycles'.
A tank that size can't handle the 14 fish you bought - even if it was fully cycled. If the shop sold you both the tank and the fish then they should have known better :evil:
Most likely the fins were 'burned' from the excess ammonia from the fish that the ugf couldn't cope with. Also the tetras would eat the fins on the Siamese fighter not the other way around. Neons can swim a lot faster than fighters.
Your remaining fish may or may not survive their experience but keep up daily water changes to help get rid of the excess waste until the tank cycles properly. Do you have any test kits? I have some here and am happy to help if you live in Blenheim. Let me know and I will get you on the right track.
Are you in Blenheim? Our club meets tonight and you are most welcome to attend. I am el presidente
18 Aug
7.30pm
Flat 1,88b Scott St (back flat)