How many fish can be in fresh water?

rule of thumb is 1 inch per gallon but it depends, If your planning on filling your tank with small guppies or tetras,zebra fish or any small thin fish you can get away with more but if your dealing with gold fish,barbs,mollies or any bigger fish i would stick with the rule and dont listen to local pet stores if they seem to be pushing them on you. their more concerned with making sales then seeing you succeed