Right... one thing at a time I guess
Firstly, a note on your choice of fish:
Goldfish are coldwater fish and each goldfish needs 20 gallons to itself. They are not compatible with the other fish you have listed and are not suitable for your hexagonal tank. Goldfish grow to 12 inches.
Angel fish require a tall tank which your hexagonal tank is. HOWEVER, a 30 gallon is reccomended as these fish do get to 6 inches.
Bala sharks are schooling fish that get to 12 inches. Like the goldfish and angels, this tank is WAY too small for them. Three balas is the minnimum number to keep and they require a 100 gallon tank - at the very least.
The pleco is probably a common plecostomus, is this is indeed the case, you are dealing with another unsuitable inhabitant. They can get to an incredible 18 inches and are messy, like the goldfish, so require a MUCH larger tank.
The cory cats are fine
However, they do best in groups of 4 or more, are very social fish and don't do well in an un-cycled tank...
Which brings us to cycling.
Considering your choice of fish and the fact that you added them all at once, I suspect you are new to this and have not heard of cycling. I would start by reading through the link you will find in my signature.
A quick overview:
Cycling is when ammonia, found in fish waste, is converted to nitrIte and then to nitrAte by certain beneficial bacteria. This cycle is essential for a healthy tank and fish.
Ammonia is extremely poisonous to fish, as is nitrIte and both need to always be at 0. You should buy test kits for all these parameters and monitor them to make sure, once your tank is cycled, that they remain at 0.
During the cycle (which begins when you add fish or ammonia in any form) you first get an ammonia spike. This means the amount of ammonia slowly increases until the bacteria begin to grow that convert it to nitrIte. Then the ammonia will drop to 0 but the nitrIte will have begun to spike. Soon, more beneficial bacteria will colonise and multiply in your tank that convert this nitrIte to nitrAte. The nitrIte will then gradualy drop. Once it has reached 0, your tank is cycled and safe for fish.
NitrAte is not as dangerous as ammonia or nitrIte. By testing your water, you can monitor it and make sure it remains below 40ppm. If it rises above 40, you know it's time for a partial water change with DE-CHLORINATED water. Changing about 25% of your water weekly, and replacing it with fresh water from your tap, of a similar temperature, that has already been de-chlorinated, should be enough to keep nitrAtes under control.
The beneficial bacteria grow in your filter and in the substrate. As such, you should not change your filter media unless absolutely necessary. Even then the floss/sponge should be removed bit by bit and replaced gradualy so the beneficial bacteria get a chance to re-colonise the new media. Ignore instructions by manufacturers that state you should change the media more often. This is only their way of making money and unecessary. If your filter becomes clogged up, take out the media and rinse it in water taken from your tank. NEVER use water directly from the tap. The chlorine will kill the beneficial bacteria and mean you will need to re-cycle the tank.
Vacuum your gravel during each water change to remove waste from the substrate that the filter has not taken out already. You'll find a gravel vacuum at your local fish store if you don't have one already.
Also keep feeding to a minnimum - especialy seeing as your tank is SEVERELY over-stocked and has not been cycled.
If you want to save your fish and continue to keep fish in future, return the remaining bala shark, goldfish, angelfish and pleco. They will never be suitable for your 18 gallon even when it is cycled.
You are better off returning the cory cats too. You can get more once your tank has cycled as they are ncie fish for a small community.
For the moment, you should have a tank and no fish. Then begin a fishless cycle. In this way your tank will be fully prepaired for fish and you'll get a chance to research the species you like and only buy those that are suitable.
That brings me to fishless cycling...
Fishless cycling, obviously, is cycling without fish. The best way is using pure ammonia but you can use fish food or a frozen prawn if necessary.
What you do is buy a test kit for ammonia, nitrIte and nitrAte. Then add some ammonia to your water. Say you add one tablespoon, now test for ammonia. If the number is about 5ppm, you can keep adding one table spoon of the ammonia daily. If it's less, add another table spoon and so on until you reccord 5ppm on the test kit. Add that amount (for 5 ppm) daily from then on.
You'll see your ammonia increase daily as you add more and more until it has spiked. Eventualy, bacteria will grow that will convert the ammonia you add daily to nitrIte. Keep adding the same amount of ammonia daily and test your water for nitrIte. As you don't have fish, there's no need to worry about water changes or anything. Keep adding the ammonia until your ammonia and nitrIte read 0. Keep adding ammonia until you are ready to buy fish. Your nitrAtes will now be very high.
Just before you go get your first few fish (never add lots at once), change your tank's water. Remove about 90% of the water and replace with de-chlorinated water. Test for nitrAtes, they should be below 40, below 20 is best. If not, change some more water to dilute the nitrAtes further. Make sure you get your fish on the same day as the bacteria will die out without food (ammonia or fish waste). Stop adding ammonia once you've got fish - their waste will keep the tank cycled.
You can add quite a few fish in the beginning if you have fishless cycled. HOWEVER, to be on the safe side, it is best to only add a couple at a time. Having fishless cycled, you may wish to add 2 cory cats. The following week, another 2 can be added. You may then want a trio of platies - you can put those in a week later and so on until you are fully stocked.
A good guideline with stocking is one inch of adult fish per gallon. That means that your tank can hold 18 inches of fish. But you have to use the adult sizes. Cory cats grow to 3 inches but the inch per gallon guideline is relaxed with them cause they are considered to be only half their size due to the tiny amount of waste they produce. None the less, don't put in 12 cories! the space simply isn't enough.
What I'm trying to say is that the guideline is only a GUIDELINE. There are exceptions but the bala sharks, goldfish, angels and pleco are not one of these exceptions and deffinately fish you need to return to your LFS. They can hopefuly exchange them for store credit.
I'm not sure if there's anything I've missed... I also suspect people will have already beat me to it and answered your question but nevermind
Good luck and feel free to ask anything
Oh yeah - and read the link in my signature (just incase you forgot that I mentioned it earlier) and read through the pinned topics in the beginners forum.
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