Some of my fish are dying (or getting killed) :(?

There is serious stocking issues here. Your fish are pissed off and are conducting night time ninja wars for supremacy.

I know it's super hard when there are thousands of different fish to choose from. But you have to do it- try stocking the tank appropriately, then when you have the overwhelming urge to put more fish in your tank get rid of some...

Your Ghost black knife fish are predatory and could be stressing fish out at night. They are nocturnal hunters . If you wish to keep this fish to adulthood you will have to consider different stocking ideas. Here is a link that has all information about this fish, even tank mate ideas.

http://aqualandpetsplus.com/Knife%2C%20Black%20Gho...

Your tank is 55 gallons. A few stocking suggestions would be.

1 large school of mid dwelling fish like a barb or tetra.

Bottom tank schooling fish like corydalis.

A couple larger "center piece fish" like a Gouramis or a couple Roseline barbs.

Or,

2 beautiful Angelfish

4 Ram cichlids

a school of corydalis

Perhaps a school of 6 tetra

Or,

6 Rainbowfish

2 Roseline barbs

A school of emerald green corydalis

Or,

A school of 6 -10 tiger Barbs

1 redtailed shark

loach species for the bottom

For a Pleco I wouldn't get anything larger than a Bristlenosed Pleco (5 inches).

Your tank now is full of mixed match fish. This is causing stress in your tank and is naturally balancing itself out. You may have a single fish pestering others at night when the lights are out. This could be any of the fish on your list besides the tetras.

All the tetras should be in groups of 6 or more, if not they will start attacking and harassing other fish. They are schooling fish and if they can't interact with their own kind, trouble starts. That is why all the sites say. "6 or more of the same species". That is why you have only one neon left. They are the smallest and most vulnerable. Get rid of the Barb!!!!!!!!!!!!!! #1 suspect and the sucker if it's an algae eater species. #2 suspect.

http://tropicalfishandaquariums.com/Tetras/index.a...

The only possible suspects in the way of fish in that list are the Ghost Knife fish, the Tiger Barbs and maybe the sucker fish, depending on what type of sucker fish it is. Tiger Barbs are usually quite aggressive and could easily be the culprit.

With the regular maintenance that you are doing it is less likely that water quality is a problem. However, it wouldn't be a bad idea to check your water. The main tests that you want to be doing are pH, Ammonia and Nitrite.

As you said, these are known to be docile community fish. Maybe your fish are dying because the water was not cycled fully. You said you were a beginner, and that is an easy mistake to make. Also, make sure you have a proper filter for that size aquarium, a bubbler, and a proper heater.