why do my fish keep dieing?

Question
I have a 55 gal freshwater tank. I'm always checking the levels of the water,
doing water changes, have a special filter that hooks up to my faucet that
takes out ammonia etc. when i go to refill the tank, vacuum the gravel,
change sock over filter that captures the waste. I keep the min of fish allowed
in tank and yet I have no luck keeping these fish alive.I only feed every other
day to every third day. I feel horrible about not being able to keep the fish
alove, as I'm a huge animal lover.  When I go to check levels again and they
seem fine, I go and add more fish in to replace the ones that were lost,
especially the schooling ones and hope for the best.  I only put in   
community tropical freshwater fish in my tank. e.g. assorted colisa Ialia,
assorted mollies, assorted platys, and assorted tetras. I have a heater in my
tank to keep it a steady 70-75 degrees. When ammonia seems to be reaching  
just a little over the safe line, i'm always adding the safest chemicals to bring
it back down. I don't know what else to do. Please help me and give me any
advice possible. What else should or shouldn't I be doing? I don't clean the
filter too much, just enough. I do have decorations in the tank, but clean
them well and I haven't added anything new to the tank. Please help.

Answer
Hi Robyn:  Feed your fish twice per day... what they will eat in three minutes of time.  Give them a varied diet of flake food, brine shrimp or tubi-flex worm (live or frozen)  Change 10-15 % of the water each week and gravel vacuum the gravel when you change the water... You can also bump up the heater to about 78 degrees.  If your water chemistry shows ammonia then clean the gravel and do a 15% water change....  hope this helps... dave