filtration system for 5x18"x2

Hi all,

I'm currently planning for a Gold fish only tank of 5x18"x2 size tank
(18 to 19" dept only due to constrain of exisiting build in cabinet dept). I'm wondering which type of filtration system is best for this type of GF species tank .

Thinking of 8 to 9 GF ( species not decide yet )

1. Sump - (this will need some modification to my existing Cab and is do-able, need more advice )
2. OHF - ( will this be sufficient ? )
3. Canister - (same as no.1 above )

Would like to have min. item in the tank, as this tank gonna be like display GF tank at home free of all clustter. ( wanna hide most thing away from main tank)Thanks

Regards Softice