I have a 20 gallon tropical tank. Eventually, I plan to have these fish in it:

5 zebra danios
9 neon tetras
4-5 cory catfish
= A total of 18-19 fish

If I run two Whisper 20 power filters (which pump 105 gph each= 210 gph total), will that be enough filtration? I do water changes every other weekend. Thanks!

You've got plenty of filtration. Actually one Whisper 20 is enough if you stay on top of tank maintenance. Personally, I would go with either just one Whisper 20 or a Whisper 30. Not set in stone, tho, so as long as the fish have a place to rest, you are fine.

Daniel

with a tropical tank setup, you don't necessarily have to follow the 10x filtration rule.
the 10 times turnover rate is for goldfish tanks since they produce so much more waste than most other fish.
with your 20 gal tropical tank, you can just use one of the HOB filters like fishieperson stated.