How Do You Maintain A Healthy Running Fresh Water Fish Tank?

Well the 1st things to consider are : What size tank do you want, (smaller tanks are mostly better for common fish like goldfish and mollies, and in bigger tanks you can more exotic fish) what kind of fish do you want, do you have the time to care for the fish.

If your a beginner i suggest starting out with something like a 10gal. Tank, and get about at the most 7 fish (depending on size) Add water to it and let the tank run for about 1week BEFORE adding fish.

Then once the fish are in and seemed like they have adapted to the tank, just leave them pretty much alone. If you have tap water you can buy chemicals to add to the tank to replace the lost minerals and other things that tap water and other water lack, (make sure you use the recommended amount). To clean the tank - its NOT best to replace all of the water because your fish will probably get sick (usually they wont swim a lot or hang around the bottom of the tank, until the water develops chemicals to fight bacteria and other things) .But to replace SOME of the water (which is at best, to leave sitting at room temperature for about a day before adding it to the tank)
You can also buy special vacuums that clean up the droppings of your fish and the algae that may form along some of the rocks or decorations in your tanks