I always use two, as do most other people. If you are going with the AC filters, I would do two AC 20s or one AC 20 and an AC 30.Ok how many filters should I have for a 20 gallon tank?
others will benefit with the questions you ask here.
this is the 'newbie' forum..the only dumb question is the one that doesn't get asked.
re how many filters..
I agree..2 first this will allow for redundancy. next it offers the opportunity for better water flow.
having 2 filters also helps minimize 'dead' spots
Having 2 HOB's also helps keep the bio media in the filter when cartridges are changed. You should always change the filter media on an alternating schedule. so one filter always has established media in it.