Dead Fish...

I have a new aquarium (about 3 weeks old) that I am having trouble with. It is a 30 gallon tank with a whisper 30 filter and a whisper 60 air pump. Inside the tank are two decotration pieces (ships) that have fake plants. I followed all the directions for tank setup and waited about 3 days before adding 3 Platys. 2 days later 1 platy was dead. I cheacked water quality and had virtually no ammonia or nitrites. A week later I added a Green Tiger Barb. About 4 days later he started to get some white cottony looking stuff on him and looked like he was having trouble breathing. 2 days later I pulled him out and put him in a medicine tank and tried adding some anti-fungal treatment. He died that night. I then added 4 more Green Tiger Barbs. The next day they also started to get the white cottony looking stuff on them, stopped eating and became lethargic. I also noticed my lone Platy now too has white, cottonly substance on her, but she is still eating fine and staying active. I went ahead and medicated entire tank with anti-fungal, but 1 1/2 days days later all 4 new Green Tiger Barbs were dead. I just went ahead and did a 50% water change and added 6 more platys to the tank. Are they going to die also do you think? I've done 2-3 15% partial water changes since I got the tank, feed very lightly, and all my equipment seems to be functioning fine. What can I do? Thanks for your help!

Also I have a tank heater and the temp of my tank is pretty stable between 78-80 degrees.

I am using liquid test kits, but I only have 2 kits... Ammonia and Nitrite. (Both bu company called Nutrafin). I have used them several times and have never registered anything significant. The test has you compare based on the color of the water and the chart on the back of a little book supplied with the test kit.

I followed the "fishy" method. I only put two fish in there originally. Once one died I figured I better put 1 more in there to get a little something going. Once he died I asked the pet store and they said I need more fish to get the cycle going. He recommended I put the 4 Green Tiger Barbs in there so I did. Now they're dead... I'm in no hurry to cycle. If one fish is enough to get the cycle going I am ok with that. Now I'm just concerned that I may have a fungus and my platys are going to die too. I'll do partial water changes everyday if that is what it's going to take. I'll put more fungicide stuff in there too if I should. Trust me I really don't want to keep killing fish.

As far as food goes...

I have been feeding a flake about the size of my pinkie fingernail for each fish 2x a day. I watch them for about 3-4 minutes and the gobble it up pretty fast. I don't think I am overfeeding them, but maybe I am?

Water quality...

The water has started to have a little "snow storm" effect. It is a combination of super tiny little bubbles and what looks almost like dandruff. Looks this way all of the time. Right after I did a water change and even two days after a water change. Like I said I have done a 15-20% water change approximately every week since getting the tank.

After adding the chemicals the water is 99.9% clear which on the scale is .1 or less of nitrite and maybe .6 MAX of ammonia.

The other thing is... My original Plays it obviously not feeling well. She is very pale, has clamped tail fin full of dandruff, and her dorsal fin is almost non-existent.