Help my fish keep dying!!!

[cezza44]

Alright so i have had my 70L tank for about 3 months now. It's a all with on tank with added powerhead and heater (set to 25). I originally got it, set it up with a couple of plants and rocks, then left it a day to work its stuff out. The next day i added a couple of neons, a bristle nose and a snail. From then on I added some Gourami's, fighting fish, a ghost knife and loaches over a space of about 2 months.
Then rather rapidly I lost two gouramis and 2 clown loaches. I went to a store with a water sample and they said i need some salt to lift the hardness of the water. They told me to dump 7 heaped tea spoons into the tank. I did so when i got home, and when I awoke the next day I had about 5 less fish.
I then to a different store and they said to never add salt and that i just added to many fish and the biofilter wasnt able to cope yet. So I added the stuff they told me (an ammonia filter), and the tank seemed to settle. I have been slowly dropping the PH, as i was told it was too high.
After about 2 further weeks i bought a new bristle nose and snail. But after less than a week a female fighter is dead and so is my favourite snail (Albert).
Could somebody please help me