Filters?

I love my aquaclear filter. I have what was called a 150 on my 20 gallon tank (lots of plants and very few fish). IThey have changed the name but I would suggest getting one that they rate for a 40 gallon tank. A little extra filtration is good. The filters with biowheels I have found make a little more noise. Maybe not for others though.
 
So far they have been great for me. They have large holding capacity so you can add an extra sponge.

The main trick is to make sure the water level is at least at the edge of the outtake. That way you won't get a splashing sound.
 
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