aquarium ph to low and wont change!?

It all started when one of my fish had fungus. I put fungus medicine with the filter cartridge( i know im not supposed to but i was told too.) After two days of using the medicine my water got cloudy and my ph dropped to 6.0. I have platys and pletcos and i know they cant handle such acidic water. I dont think it was from over feeding though i did feed alot two weeks ago. Even if it was from over feeding , after two water changes in a week i think the ph should of adjusted by now. Yes i know to only take out 25% of the water. I called tetras hotline and they told me to use aquarium salt and tetra safe start because they think my bacteria is out of wack. Petco and tetra said that it might be my ammonia but my ammonia level is normal. PLease help me. Its been a week and im scared my fish wont survive. 20 gallon fresh water. has 11 fish



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