Fresh Water Aquarium/Dying Fish/Baby Fish

Question
I am trying to figure out why I am having problems keeping my fish alive.

TANK HISTORY
I originally set up my 30 gallon tank not quite a year ago.  I had a 9-year Bala shark that I took from a friend moving away.  The shark lived only half a year with me before it started swimming upside down (I was told swim bladder disease).  So i kept replacing water and left the tank and plants running for a number of months until in February I decided to start up a new tank.

I added a new bala shark and 2 other fish (green striped tiger?).  Within a week, one of the other fish died and I bought a replacement and 2 platties.  When the next fish died, I went back to the store and had my water tested.  They said I had high Ammonia.  I used a gravel vacuum and added more 'Cycle' active bacteria to the tank to try to help the situation.  Within a few days all I had left was the shark and the two platties.

They all lived together for a few weeks until the shark got ick.  I bought a treatment and followed the directions - adding the proper amount and I raised to the temperature to 81 as directed.  All the fish seemed fine during the treatment until the shark died the day the treatment was finished (last week).  So I was left with 2 platties.  Yesterday I noticed that I have a third baby platty.  However today one of the parent platties died.

Please help.  I just want to get my tank balanced so my fish will survive.  Is there anything special I can do to ensure that the baby will survive?

Answer
Green Striped tiger barbs... I think that is what you bought....

What I would do is this:  I would go to the aquarium and ask them for "dirty water" it is basically used water that is squeezed out of a sponge filter.  It is fully loaded with beneficial bacteria for your tank.  

The problems is/was this:  once your tank cycled and became balanced... you added new fish.  Fish are both a biological and chemical contribution to your tank water and the cycle became unbalanced.  Think of tank water as its own creature.  When it is balanced it is healthy but when it is unbalanced it becomes sick.  When the fish became sick you treated the tank water with medication to help the fish and the medication killed off all of the bacteria ... good and bad... and the tank had to cycle again.  In order to use the medication properly you have to remove the filter cartridge and thus you have nothing in your tank to stop the build up of harmful chemicals such as ammonia.  This is why the fish died. What you have to do is this:

change 20% of the water weekly, add aquarium salt per its instruction label.  Replace the filter cartridge and media.  Keep the temperature between 78-82 degrees.  Optional:  I would add in about 3-6 live plants.  Platy's like algae and they love to pick the algae off the plants.  Plants will also help to filter your water of ammonia and nitrates.  Please keep me posted... dave