fish dying, seem sick

History:
I inherited my tank last Dec with 2 goldfish....they got stressed and died of ich in April. I had added anther goldfish and a weather loach, also a snail (non-reproducing). The weather loach and snail were the only survivors. Afterthe meds had been cleared from the tank for about 2 months I got 3 neon tetras, 2 albino catfish, 2 red platys, and 2 ghost glass cats (you can see through them), all added slowly over a month time frame.
Problems:
First off, I have brown stuff growing in my tank. The fish store says I am overfeeding, but I am feeding the fish less than what the directions say, and the devour the food within 2 minutes. I live in a rural area with well water (nothing added) with a high iron content. Some people around me think the brown is from the iron. Could this be the problem?
Second, last week my snail died. I have no clue why. One day it was fine, the next it was laying on the bottom of the tank and on the 3rd day the fish had eaten poor Slowpoke.
Third: Around this time one red platy seemed to have gotten a bit out of him, and he was missing some of one of his gills. He lasted aweek and then died. Then a neon tetra seemed to be a bit "off" and died. Before he started floating the others ate him. The other 2 neon tetras seem a bit "off" now. One seems almost bloated. They and the loach seem to be a little grayish/brownish on the scales (though maybe I am parinoid).
Tonight the loach was hiding (unusualy behavior for him, if he doesn't get fed on time he swims fast, up and down-once he jumped out of the tank, I got a different lid since then-until he sees/feels the lid open and gets his food) and as he was eating the food he took into his mouth came out his gills! I have never seen this happen before and we watch him eat all the time!
Can anyone help me?

The fish store said that the snail would eat the leftover food the fish did not eat and that it did not eat plants, only algea. Already I have a bunh of green stuff growing since it died
I do a partial water change every 1 to 2 weeks depending on my business and how the tank looks. All the test are normal, ph at 7 (I have to add sodium nitrate because our water is a 9) no ammonia. i have a test kit (adding drops, not strips of paper) and I test it before and after a water change. I vaccum the gravel each time I change the water (only a quarter of the tank at a time). Oh, and my tank is 25 gal.

oops, I meant that I do the water change according to how busy I get (forgetfulness)

Yes, i bought a heater and have the temp set at about 82

What type of vegies do you feed a snail? I will go do water tests this morning to recheck levels because I cannot rremember the nitrite or nitrate levels.

Nitrite is below 0.3
ph is at 7.0
ammonia is at 0.0
I do not have a nitrate test. I guess I have to get one tomorrow. Trouble is I live 1 hour from fish store. What do nitrates do?
Oh, on water conditioner:
I use Cycle and Stress Coat. Because I have well water (no chlorine or anything else added) I don't have to get the stuff to take chlorine out. Should I use something else?