You'll need more than a 20 gallon sump for that. If you don't want to go the sump route... and HOB's won't work... a couple large Canisters will do just fine. Tthere are many that will work for your needs... like the FX5 or rena XP3/XP4, several eheim's like the 2217, 2260 etc. etc.. If it was my tank... I would probably just go with two RENA XP3's or two Eheim 2217's.
Congrats on the 300!
- Jonathan