Help There's A Killer In Our Aquarium

My roommate has a 30 gal tropical freshwater tank. When we first moved in we had 22 fish and two blue lobster who ate 3 Bala sharks and a few orange mollies and some rainbow fish... basically had a tank of 22 and it went down to a tank of 16. We got rid of the lobsters and the fish store gave us credit so we got some more fish the other day. Since then there is a killer in the tank and I need help. We have:

1 bala shark
1 molly (and her baby)
4 black tetra
1 Pearl Gourami
1 three spot gourami
2 Long fin swordtail
2 angelfish
2 Danios? (WHITE FISH that look like female swordtail fish but not orange) [picture of white fish attached]
4 orange and black swordtails

killed:
1 rainbow fish
1 Danio? (WHITE FISH that look like female swordtail fish but not orange)

anyone think they know who killed them?

please help. its not my tank but i hate waking up to a dead fish.

Possibly the Bala Shark or the Angelfish could be the killers. However...

You need to test the water for ammonia and nitrite using a liquid test kit and give us readings. Your fish could be getting poisoned by a lack of vital bacteria on filters and inadequate maintenance of the tank.

I'd go as far as to say that once you have enough water sample to do the tests above, remove 75% of the water and replace it with fresh de-chlorinated water of a similar temperature pronto, as in right after you read this sentence.
Try and gently rinse the filter sponges in the removed water and gravel/sand vacuum the tank floor while removing water from the tank.

Even if the readings are good, fresh water often purks up ill fish.

Have you checked the fish for signs of external illness like white or yellow spots, fungal growths etc.?

You also need to get rid of the bala shark. He will get way to big for that tank (actually need 5 foot tank min as they grow to great than 12 inches) and they like to be in schools. Other than that, you look a little overstock there. Don't buy anymore fish. This could be fouling your water and causing the fish death.