Filters:
The first thing to remember is that these are river fish. Enclosures should have a lot of moving water, but you'll also want areas of less flow. With systems the size your talking about for an arapaima, you're going to be looking at at least one good size sand filter, possibly a large carbon (if you are so inclined), a large bio tower (or sand bed if you can accomidate one) and possibly ozone. You'll also need pumps up to 5 or 7 hp, lots of PVC (sch 80 -NOT sch 40), ball valves and check valves, flow rate meters, pressure gauges and a host of other plumbing basics.
**As for ozone, I'd suggest not going there unless you have the training. Large scale ozone units are a quick way to kill yourself. **
Most of the things you'll need are availible as pool filters. From there, it's matching cubic feet to the amount of water you have. Good flow (depending on tank volume) should equate to 100% turn over rate two or three times an hour. You'll want both a surfce skimming suction line off of an overflow box as well as a bottom drawing suction line. Both can feed into a pump which will push water into the filter and back to the pool. If you are using several types of media in various filters, several pumps my be needed. Also, I'd recommend having a single smaller pump running just circulation (straight from and to the pool). this is great for adjusting flow and use in case you need to take a filter off line for servicing.
Most sand filters are round for this size application. Thse work well for saving space and maximizing filter surface area. If you look into an elongated tube style, keep in mind you'll need more water to fluidize the bed completely during a backwash. Elongated beds allow more surface area for water contact. These style are usually used on extremely large systems.
Depending on where you're located, you'll want a heat exchanger as well as a chiller. Some areas, you'll need both to maintain proper temps.
