Help with my 5 gallon tank?

I have a 5 gallon tank that I bought in April. I didn't read up on aquarism beforehand unfortunately, and I have had no luck cycling my tank. My ammonia levels are still always through the roof, despite my twice daily water changes. The nitrate and nitrite levels are still 0 (undetectable by my API master test kit). I have one sunset platy Left out Of all the fish I originally bought. Why won't my tank cycle? I thought low oxygen conditions might be to blame, so I bought an airstone and air pump. No help. I then bought an additional filter, which accomplished nothing but sucking up my 3 corycats and killing them . I am at the end of my rope, what can I do ? I use dechlorinator to make the water safe, and I add seachem stability with every water change . I vacuum the gravel (about 1/3) every few days.