I'm kind confused, haven't you had these fish for a while? Changing the filter could cause real water problems because the beneficial bacteria that convert ammonia to relatively harmless nitrAtes live there. Anyone keeping fish needs to have a test kit of some kind. You can't judge water quality by looking at it and high levels of ammonia or nitrItes can cause the fish to come down with all kinds of problems due the weakened immune system.
Water changes are the only way to remove the nitrAtes and other chemicals that accumulate over time. It is also the way that you put dissolved minerals back into the tank. You should change 25% of the water each week once you know that the biological filter is functioning, until then you need to get your water parameter checked at a fish store while you are there getting your vacuum. Don't accept vague responses from them, get actual numbers for the ammonia, nitrItes, nitrAtes at least. Please purchase a liquid test kit such as the API Master Freshwater kit. The liquid tests are much more accurate and much cheaper in the long run than test strips.
Make sure any new water you add is treated with some kind of decholorinator such as Prime by SeaChem. Make sure it is close to the existing tank temperature when you add it back in. You will remove water by vacuuming the substrate. If you have never stirred the substrate, be careful as you could stir up some nasty anaerobic bacteria, it stinks like sulpher and can harm the fish. Just disturb the top inch or so.