Some of my fish are dying

Question
I went out and bought
2 Dwarf Gourami
10 Neon tetras
2 Black Neon Tetra
2 gold fish
2 Bala Shark
I have had all these fish together for about five days and yesterday I found that my 2 Back Neon Tetras were missing.  Then today my parents said that they found one of the goldfish mauled to death along with 2 neon tetras dead.  Just now I have found another neon tetra dead and I am wondering what is killing my fish.  I thought it would be the Dwarf Gaurami, since I have never had them before, but every review I read says that they are very tame fish that are good with communities.  So I was wondering if you had any idea as to what is killing them, this way I know how to fix the problem.  And if it is a fix I know not to buy them again.
Any thoughts would be greatly appreciated.  Thanks!

Answer
Hello Olivia!

Since you have 18 fish you would need a really big tank especially since you have Bala Shark {They can grow REALLY big}. You would be looking for a 55-75 gallon tank. It can be the Bala Sharks because they are semi-aggressive so they may chase and nip other fish.

Another reason could be the temperature. If you just bought them and put the fish instantly to a prepared (Having everything set up, decor., soaked/rinsed gravel/sand) tank/aquarium, they will become stressed out due to extreme temperature change. It's like going in a hot tub then jumping into a cold swimming pool. Try getting a larger tank.

Hope this helps! :]
~Sam