No idea. Can you post pictures of the tank including the filter?You answered about everything I could want except for one thing. With built in filtration is that included in the 20 gallon tank size or would the 20 gallon size just include the actual tank area not including the filtration? Not really sure of modern terms but my guess is that my tank would be considered a nano.
As a general rule if the filter is inside the aquarium, it will be full of water so just measure the aquarium as described above.