Check out this video for an idea of what it takes to run a tank/pond large enough for an Arapaima:
http://animal.discovery.com/tv-show...eos/most-outrageous-largest-home-aquarium.htm
He's into his setup for $100K. You can do it for cheaper if you don't do viewing windows, but I would estimate it's still going to cost you around $50k.
The monthly expense of running a tank this large is also astronomical. He's running a total of 24KW heaters (that's 24,000 Watts). Even if his heaters only come on 4 hours per day to maintain temps, that's over $315 per month (assuming the national avg of of $0.11 per KWHr) just to heat the tank. You are in CT?...it gets a lot colder there than in Southern CA where the tank in the video is located. Add to that the cost of running pumps large enough to have adequate filtration and you are looking at an electric bill alone of at least $500 per month. If you plan to setup the pond indoors, you may save some on heating costs, but you will run into serious issues with humidity. To set up an adequate climate control so that your house doesn't end up with serious mold issues will offset most savings in heatings costs that you would gain by keeping the pond indoors. Water changes on tanks/ponds this size also do a number on your water bill. Even if all you keep is the arapaima, the cost to feed a full grown specimen is going to cost you $100+ per month. So to say it costs as much as a kid is not off base.
Let's break it down:
Monthly Cost = ~$700
Cost Per Year = ~$8,400
Cost for 20 year life of fish = ~$170,000
Cost of setup = ~$50,000
Total all in cost = ~ $220,000
The cost of the fish is just .0009 (.09%) of what it will cost to keep.
My apologies if you have taken all of this into consideration.