lots of questions!

Question
my tank is quite small around 25 litres/ almost 7 gallons..i change the water every month..tank is around 6 months old..i dont use any ammonia nitrate stuff i donno what its all about..i have the follwing fish in it:
1 molly
2 glass fish
2 tiger barbs
1 bloodheart tetra
1 angelfish (2 angels together fight, right?)
1 Siamese fighter (2 fighters also fight? i've been told)
1 sheban king
1 rainbow shark

now i've noticed that the rainbow shark has been nipping around the fins of the tiger barbs..though the tiger barbs have not lost any fins..so do rainbow sharks attack barbs? is the combination of fish in my tank compatible? i also added a second molly and a catfish/sucker recently and they both died within two days..what could be the reason?..also one of my barbs has one eye swollen..whats the reason..the eye is still intact and its quite active and it can see well but the eye is still swollen...also one of my angel fish's fin is damaged and im wondering if the tiger barbs did it since they have teeth tought i have never seen them attacking any of my fish..i also want to add a carp or cory to my tank..any suggestions?

ps: can u please email me the answers to zohaib_abbasi@hotmail.com cuz i found this website through google and i doubt i will ever find this page again!

Answer
Hi Zohaib
It should go through to your hotmail account automatically once I send it, so you don't have to search through google again lol.  I'll keep an eye, if you don't read this within a day I'll send it to your email.

Well, to start, that ammonia and nitrate stuff is important to know.  It's not something you use, it's something that occurs in a filtered fish tank.  It's called the cycle process.  Here's a link with an article that explains the cycle process, there's also an article on ammonia and nitrite poisoning in fish, that are important to know as well:

http://freshaquarium.about.com/od/startupcycle/Step_5_The_Cycle.htm

Read through that and let me know if you have any questions.  You also should pick up a test kit, that tests for ammonia, nitrite, nitrates, and ph so you know what those levels are in the tank.

Now, you're tank is serously overstocked-too many fish.  And you have a lot of incompatible fish, and just not good combos.  I do not recommend adding a cory or a carp to the tank, or any other fish.  In fact, I'd recommend removing some of the fish you currently have in there.

I'll just go down the list:

Molly-fine in there

Glass Fish-fine in there

Tiger barbs-I'd get rid of these.  They prefer to be in groups of at least 3, and then I think your tank would be pushing overstocked.  Plus, they tend to be fin nippers.

Bloodheart tetra-get rid of this.  They prefer to be in groups of at least 3 like most tetras, and 3 would be too much for your tank with other fish that you could keep.  

Angelfish-get rid of this. Tank is too small for this guy, they need at least a 20 gallon for 1 of them.  

Siamese Fighter/Betta-I think you can keep this one.  Should be fine with the molly and glass fish.  Yes, they will fight with others of it's kind-male and female.  Only 1 of these to a tank.

Sheban king-I have no idea what this fish is, it's probably given a different name wherever you're located at.  Tried doing a search, no relevant results to a fish though lol.  I'd say probably get rid of it based on the fish I recommended you keep.

Rainbow shark-get rid of it.  Grows too large for your tank, and have a tendency to get aggressive/territorial with other fish, not a good community type of fish.

So, I'd recommend keeping the molly, the glass fish, and the betta/Siamese fighter, and that's it-no more for your tank size.  Or, you could keep most of those if you upgraded to a much larger tank, like at least a 30 gallon/114 litres.  But since I don't know what the sheban king is, that may or may not work in that size tank also.

Tiger barbs and the shark are known fin nippers.  I'd guess one of them is what nipped the angels fins.   But, because your tank is overstocked, and I'm going to guess the water quality is probably bad, it could be fin rot on the angel fish.  The water quality being bad would also be my guess as to why the molly and sucker fish died, and the cause of the swollen eye on the barb-that sounds like a condition called popeye, but that could also be from an injury.

Read through that link about cycling a fish tank, again, that's important stuff to know if you want to successfully keep your fish alive.  I'd strongly suggest getting a larger tank or getting rid of those fish I listed.  Also, in the mean time till you do one or the other, do a large water change, at least 50%, and do that every 2 or 3 days until you get a larger tank or get rid of those fish.  
Let me know if you have any more questions!  And good luck!

Christy