I am planning on buying some African Clawed frogs in the near future and have learned that they do not like a strong current. What I can't find is a solid answer on whether filters are completely wrong for them or are a-okay as long as the current is low. I was planning on getting a hang on back type filter with an adjustable flow and would aim it at a tank wall as best I could. If that didn't work by itself I might find some other way to help diffuse the flow. However I also heard that simply the noise from the filter can cause stress for the frogs. So I am not sure what to do.

Do you have any filter suggestions? and if you do not suggest any filter be used how often should I change the tank water and how much of the water should be changed? I will be using a 20 gallon long tank and plan on having three frogs in it.