Have you stopped the Excel? Being a low-tech tank, you should use a lot less of any fertilizer. I have a 29G I add 1/2 dose (5ml) of fertilizer to my tank once a month. Too much fertilizer can pose a health risk to your fish.
Do you know anything about your filter? brand, age, how often it cleaned and media is replaced?
Get a couple of 5 gallon buckets. Fill them with your tap water and let them sit for week, use that stall water for your water changes. Our city water (Honolulu) is good stuff but I age it before using it. I wonder if you have heavy metals (lead, copper, etc) in your water. Might be in low enough numbers not to effect the big animals, but could harming the fish.
I was thinking that maybe you should try keeping guppies, they are hardy fish and very beautiful. They are the fish are started with and in a number of why I miss them. While my fancy and costly fish are pretty, they can be so much more work. The guppies are just happy being guppies and put up with a lot of my mistakes, that would kill my stock.
Many I suggest a booK:
. I've read dozens of fish books, this was my favorite. I learned more and made better choices with this book, than the others.
Not everyone starts out with a perfect tank, where all the fish thrive, the plants grow and everything is perfect. I struggled and still do with my tanks. I finally after a month rid my big tank of algae (new filter and water near total changes). I lost two neon tetras this weekend, after adding 10 cardinal tetras (not sure if its related - but now I worried I have added sick fish to my tank). It can be a royal pain, but when I'm able to just set back and watch my tank, its one of the greatest feelings.