Sorry about total newbie question! I am reading on the fish I want in my new 38g tank. Pretty much every fish prefers moderate water flow. So... What creates water flow? Is it a filter or a special piece of equipment? If it is a filter, what types of filters let you regulate the flow? Also, unrelated question: I hear there are some types of filters with 2 slots, allowing for a partial change of the media- what brands are those?