So, my kids me a 30 gal (I assume you are meant to use the liquid gallon) tank a few weeks ago.
Came complete with filtration system, temp control, air pump and UV lighting.

They also bought some fish, some simple plants and pebbles to go with it.

I really don't know what I should be looking at and where to proceed.

We had one problem that a couple of the fish seem to be eating the angel fish!
I am not sure what the fish are called. They are orange - rather like goldfish with less fins. The male has a sword like appendage on the tail.

Anyway, after I discovered that they were the cause of the mysterious disappearance of other fish, I isolated them in a smaller tank within the main one.
The male promptly died!
I moved the female back into the main tank and it's all quiet now.

Sounds like the "goldfish" like fish are platy or swordtails. They are livebearers, and even though the other one didn't have a sword, it may have been a female. Males have a gonopodium (you can guess what that is) and that's how you can tell the difference. I'm not sure about them eating angelfish, as I have no experience with angels myself. How long have you had your tank set up, and have you tested the water to see if you have ammonia/nitrites? If not you should get a test kit so you can monitor that. I still pretty new to aquariums myself, and i'm sure that someone else will have more input than I, but you definitely need to test your water.

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

First off what strikes me as odd is how a sword tail would eat an angle fish.How big were these angels?And are you sure thats what they are?Because most stores that have angels dont let them go until the are nickle size,in which case a swordtail platy or molly either one is not big enough to eat.

Second as Summer stated,a test kit is very vital to the health of a tank especially a newly setup one.You will experience a rise in ammonia which is toxic to fish,then nitrite which is not as bad but still harmful and finally the nitrate which is harmful in large amounts.I am guessing you did not cycle the tank first and also would like to know your acclimation steps.

Many fish should be drip acclimated and even though it could seem like a pain it is a potential life saver.I am thinking its more of improper acclimation and uncycled tank that did the fish in more than the other fish eating the ones.All fish will eat a decomposing fish,its natures way to keep pollution down.

Are the plants live or fake?Live plants are best as they help the water quality.

It would be very helpful if you could get a picture of the fish and the tank so we can help ID the fish.

Sorry its started out rough,but I promise once you get over the hurdles,and everyone here will help you with that in any way we can,you will thoroughly enjoy the hobby!

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http://bountifulbettas.blogspot.com/

"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

They were definitely attacking them.
I saw them "pecking" away at them and the angels were clearly in distress.
Started with 10 angels. 3 disappeared completely and I found 2 others dead.

The tank was all set up in on shot including the fish.
I have a feeling that that was not a great idea (putting the fish in right away) but it was a gift.

BTW, about the "orange" fish, which one is the male? The one the normal back fin or the one with the sword-like extension?

really it depends.Platies look much like swordfish,without the appendage,so with the one without you will have to look for a gonopodium.The one with the sword,is all male though.

The gonopodium will not look like a normal fin.Platy - Tropical Fish Forums

Its the same for mollies platies and swordtails.

Is your tank uncovered?The missing fish may be on the floor so check for them there.Some fish will leap out especially if the water is not to their liking.

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http://bountifulbettas.blogspot.com/

"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

Ok, so I took a couple of pics.

Hard to get them to stay still.

The Attacker



The victim(s)



Remember, I am new to all this. So it I completely misidentified them please frogive.

Ok so first pic is a swordtail,female.One of the little fish with it is a black skirt tetra.I am shocked its not the opne doing the picking as they are very nippy.They will go for angelfish,especially if they are not in a school of their own.The other little fish seems to be a guppy.The little fish in the second pic is also a black skirt tetra.

Its understandable about misidentifying as a beginner,we have all done it,myself included.

Seems you do have some live plants in there.I suggest you add alot more to break up line of sight and that will help with the nipping.

Your female looks pregnant too,BTW,so you may end up with fry before you know it.

Pics are pretty good.Fish dont like to hold still and pose.Well, mine do but they are strange,hahaha.

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http://bountifulbettas.blogspot.com/

"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

So can you give a breakdown of what is still in the tank? A 30g is only suited for 1 Angel in a NON-planted tank - add plants and it changes. I wouldn't consider 10 Angels in one of my 125g tanks. They were likely in distress, or under stress, due to overcrowding and poor water quality. How do Angels disappear?

I would do a 50% water change right now. If that tank has had that many fish in it and you haven't tested your water and it's a new tank, ammonia has to be off the charts. You can get the water tested at the local store usually, if you don't have your own testing equipment. I would recommend getting your own, as was said. Don't wait to go to the store first before the water change. You have the potential to loose everything thats in it unless start testing and changing water as soon as you can.

Absolutely appalled at the fish store for selling you that many Angels at once, especially if they also sold you the tank.

No need to worry about fry in that tank. They will be little Angel snacks.

LOL,I forget about that.

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http://bountifulbettas.blogspot.com/

"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

The black skirt tetra is actually new to the tank.
Was bought after the "troubles".

With the Angels I found 2 dead "bodies" but 3 had disappeared before that.

I probably need to find a shop that specialises in fish and doesn't just try to sell what it can.

First stop adding fish until the tank cycles.You will only keep losing them.Invest in a test kit and cycle the tank.Angels sould only be one to that size tank as mentioned before.If you have more,then you may need to get a larger tank or rehome the other angels.The tetra needs a school or he will get stressed and die as well.

As it stands you will be lucky for any of the fish to actually live throughout the whole process,as ammonia burns their gills.This will inhibit the saturation of oxygen they need and thus poisoning them.This significantly cuts their lifespan down.

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"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

ONE Angel in a 30G tank?.. That defies everything I've got going on.. including 12 in a 125G with 3 breeding at the same time!... of course my tanks are heavily planted... Bill in Va.

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12 Tanks, 900 gallons. Discus/Angels/Malawi/Lake Victorias. Just added a 135 & 2 - 29's..

Blitz.. dangit.. I wish you were in my neighborhood and I could take ya under my wing... I just love spending others' money on the hobby!...LOL... hook up with a local fish club or post 100's of questions here.. you'll get 1000's of responses!.. Good luck with your pursuit.. Bill in va.

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12 Tanks, 900 gallons. Discus/Angels/Malawi/Lake Victorias. Just added a 135 & 2 - 29's..

Ok.
Changed 50% (washed the floor too ).
Now to get ammonia tester.