I don't think the 1000 gal is very far off. There has been a pretty good discussion for and against on both sides and there are a lot of unknowns that need to be addressed before anything conclusive is reached.
Tank footprint:
an 8 x 4 x 4 tank (length = 4x the fish, width and height = 2x the length) is not unreasonable to assume and that still gives us 900 gals. You can play with the numbers and even get it down to 600 gals + a sump but with the sump you're still in the ballpark (because it is still total water volume).
Bioload:
Comparing a Fahaka and an Mbu is not really going to work simply because they have different growth rates thus equates to a different metabolism and thus the bioload will be different for both fish at any given time. The ONLY true way to determine the bioload of a fish is by testing. You do the test and take your Nitrate reading just before a 50% water change and do the same thing exactly 1 week later. This will determine if your filtration is up to par and or have enough water volume. Obviously, this is ongoing as the fish continues to grow.
The YMMV component:
2 fish of the same species will experience different growth rates and different behaviors. What was good for one may not hold true for another. This is also true in genetically identical animals as proven in recent years. To say that an "averaged sized MBU is 24 inches" is also not unreasonable. Some will grow larger and some will not (given identical conditions).
Some things mentioned in the discussion that should be discarded or re-thought:
*a 500 gal with heavy filtration can be better kept than a 1000 gal with lesser filtration. -Obviously. Re compare the 2 with equal levels of filtration and equal 50% water changes with the same bioload and I'm sure you will see that the larger tank contains less pollutants.
*Trout growing in a pipe with continuous fresh water flow does not stunt the fish. - Obviously. This is because source pollution and hormones are continuously flushed out of the system. That is not so with the average tank. The only time these contaminants are removed from the system is with a water change. Attempt the same in a system where water is flushed weekly and you'll have some rotting fish on your hands.
So in essence we have a situation where some variables are up to the keeper:
More frequent water changes may allow for a smaller tank, but limits swimming space. The amount of swimming space needed is subjective to the age of the puffer and its individual temperament. For those who do weekly changes, yes, a larger thank is needed. Most of the info read on the 1000 gal tank size, I believe is geared towards those who are doing weekly water changes (which is the benchmark for determining bioload).
