Basically, yes you need to do water changes. How much depends on how many fish and how much they eat. The bigger the tank (for the same fish) the less percentage you have to change. So if you have a fully stocked 50gal tank you might need to change 50% per week. If you had the same fish in a 250gal tank you might only need to change 10% to keep similar water quality. Thats why large tanks and low stocking makes it easier to look after. Assuming all your filtering is working, all the food you dump in gets converted to waste (mostly Nitrate) that builds up in the water. If that builds up too much, your fish get sick and die
You can test for Nitrate with a simple test kit, that will tell you if you are doing enough water changes. If the Nitrate builds up above about 30ppm then you want to do more / bigger water changes. If it stays low, you could cut back, but low is good.
If you are doing regular water changes the tank water and tap water should be similar pH, but getting the water to a similar temp is probably a good idea. Water conditioner is a good idea too, depending on the quality of your tap water.
Like it sounds.. fresh water drips into the tank continously. The extra runs out through an overflow. You would adjust it so it changes enough water to keep the Nitrate level down. Makes sense when you have big tanks and fish, it can change a lot of water in a week, but no sudden changes in water conditions and almost no work. Again, if your tank isn't highly stocked you dont NEED this system, but it makes life easier if you have to do big water changes. It makes your tank like a natural pond, with a small stream flowing in and out of it.
Cheers
Ian
