FX6s have 924G/H flow rate pump. 5 of them are 4620G/H. To match your pump one needs 4 of them. FXs are not known to leak and from all the equipment I've ever used filters are the most reliable. I changed 2 leaking tanks over the last few years and my old filters are still running. FXs do not hold that much media though. They're mostly about the flow.
AQUARIUM CAPACITY | UP TO 1500 L (400 US GAL)
Item # : A219
UPC : 015561102193
For fresh and saltwater aquariums up to 1500 L (400 US Gal)
Flow Rate: 2130 L (563 US GAL) PER HOUR
Pump Output: 3500 L/H (925 US G/H)
Mechanical Area: 2,100 CM² (325.5 IN²)
Media Basket Capacity: 5.9 L (1.5 US GAL)
Filtration Volume: 20 L (5.28 US GAL)
Head Height (max.): 3.3 M (10.8 FT)
Wattage: 120V/60HZ – 43 W AND 230-240V/50HZ – 41 W
Dimensions (L x W x H): 40 X 40 X 53 CM (15.75 X 15.75 X 20.8 IN)
Not to split hairs, but the pump capacity and flow rate are not the same once you add media and head height. I based my statement off the the 563 gph and my estimated max flow for the sump of 2500 gph @ 2.75 meter head height. I am in no way knocking canisters and for my applications they are fine and have more than adequate media capacity. In fact I will probably only run my pump at about 60% and was going to suggest to
GoldFinger
that he could plumb the FX6 cans into the tank overflows since he already has them, but I still think he should have the tank built sump ready. I don't keep rays so I can't say from experience, but after seeing the extremes some go to to protect their investment and their reputation for extremely high ammonia production, I would actually use a sump and the canisters. Not that the op would want touse this, but my sump is a 55 gallon hdx plastic tote that is not very likely to leak any time in the near future.