How many fish could i have in a 20 gallon tank i am keep tetra/ platys etc?

Well the basic rule is one inch of fish per gallon of water, there are many different ways to figure out how much this will make.Before you even buy you little fish, first do some research and figure out how big the fish will be once full grown, for example goldfish can get quite big! While guppies stay very tiny. Just remember the rule if the fish you want will get large they will need more space as well as filteration because they will produce more waste. don't forget that when you add Filters, airpumps, gravel and decorations the tank will be holding a little less than 20 gallons of water, a good thing to keep in mind. Have a great time with your fish tank!