Problem with cloudy water/pleco/fish dying?

Bought a 20 gallon tank 10 months ago. Filtered water for 4 days, and it got cloudy. Added 7 small fish, still cloudy after a month. Added live bacteria to create cycle, still cloudy. Had water yzed told everything was fine. Fish were happy and growing nicely. Had 4 painted tetras, 1 scissortail tetra, 4 glowfish and 3 platys. Tank never cleared up, even after trying water clarifying drops, adding more bacteria, doing serious water changes, etc. Top of water looks like it has a "film" on it, white fuzzy stuff aculating in spongy filter and on various parts of filter which turn greenish brown when wiped off with a paper towel on fish tank wipes. Plastic plants started aculating brownish slime which I think is brown algae. An acquaintance was getting rid of a pleco (getting rid of her tanks completely), so I said I'd take one. Well it's 6" long and UGLY! It is always stuck to the side of the tank, and have been throwing one Omega One Veggie Rounds disk daily. The glowfish loved pecking at it, but I don't think the Pleco is even eating it. Since getting the Pleco, I have lost a multitude of fish: 2 newly purchased tattooed mollies (I know it's bad, but my 6 year old really wanted them), 3 glowfish, 3 platys and now my blue painted tetra. We added 3 neons a week ago, and one neon is already dead. I don't know what's going on! Tonight I noticed what looks like clumps of bellyon fuzz (for lack of a better term) with pink stuff inside it. I think it's either the Veggie Rounds or Pleco crap (this thing is crapping like CRAZY in this tank! I can't keep up with it when doing the weekly tank cleaning!) What the heck is going on? I checked the ammonia levels -- zero. I yzed the water tonight and here is the readings: NO3 (Nitrate) = 20 (under 40 OK as per dipstick instructions), NO2 (Nitrite) = 0, pH = less than 6.0 (which is the lowest shown on the chart -- this is definitely low), Alk = 0 (instructions say between 120-300 is best for freshwater) an Th (hardness) = 0 (instructions say 50-150 best). I need PROFESSIONAL responses. This is a complex problem going on, and I have consulted various professional fish people here in NYC and I still can't figure out what's going on. The multiple deaths have only occurred since the Pleco arrived. Could he be doing something to the fish to kill them? Or is there something serious wrong with NYC tap water, left over night to get the chlorine out? Sometimes we use water from the tap and add de-chlorinator to it before putting in the aquarium too. HELP! PLEASE! I only have 6 fish left (beside that stupid Pleco) and I don't want them to die too! It's been really hard trying to explain this to my 6 year old, when these fish are dying every couple days!!!