People have different opinions on this, some feel that the filter should turn over the water in the tank 5 to 10 times per hour. Others, including myself, feel that less is not just ok, but preferable. Depends on how messy your fish are, how many of them there are, how frequently your plants shed leaves, and many other factors.
Check the stats on my 75 gallon tank, and then consider that the two HydroSponges are powered by nothing more than a Whisper air pump. How many times is MY water being turned over each hour?
My personal rule of thumb is that whatever the manufacturer CLAIMS the filter can handle, it CAN for sure take care of a tank 1/2 that size.
By this standard, if Eheim claims a 2217 can handle a 160 gallon tank, it should certainly handle 80 gallons all by itself.
If Marineland claims an Emperor 400 can handle an 80 gallon tank, it should certainly handle 40 gallons or less, (and possibly larger).
If Lustar claims a HydroSponge IV can handle an 80 gallon tank by itself, it should certainly do the job in a 40 gallon tank.
This is how I choose filters for my tank, and I'm pleased with my results. My two HydroSponge IV have claimed capacity for 160 gallons total, so work very well in my 75 gallon tank.
Check my photos for a few images from a tank with nothing but two sponges slowly filtering the water.