10 gallon advice

[Leatherback1]

Ok,so about 6 months ago i bought a 10 gallon tank along with a heater,a Quietflow 10 filter,some green gravel a few plants and a cave. I added dechlorinized tap water along with a bacteria supplement.After 24 hours, I added my betta.A few weeks later i added 3 platties which ripped up my my betas tail and eventually they all died except my betta. So I do some research and decided to get 3 Cory cats.2 of them died because of bad water quality( I didn't know you had to clean the gravel).then I go and buy a Pleco because algae is accumulating.One day a nickel fell into my tank and caused my Pleco to die. I switched my betta to a different tank so right now all i have is a Cory in my tank.I plan on getting an API testing kit so that I can test my water,then cycle my tank awhile, add some endlers,cories and maybe algae eater.Any problems with my plan?Any suggestions?