Well you're not asking me but I will answer anyway - I have a refugium set up on my tank.
A refugium is essentially a sump with a special substrate to promote macroalgae growth and to release useful elements/compounds into the water to improve coral growth and fish health. Macroalgae - such as caulerpa (although it's not the best one) - is placed in the sump and absorbs nitrates for growth. you then harvest some of the growth and remove it from the system (note - feeding it to your fish does not remove the nitrates from the system).
In it's truest form, a refugium is literally a refuge for all sorts of life that will be eaten in your main tank. It is a breeding ground for all the little pods that some fish feed on, and that are an important part of the whole ecosystem.
So, if you have a sump already you can turn it into a refugium. If not things are a little more difficult as you have to have a means of moving water between the 2 - either an overflow and return pump, or drilling your tank and putting in some pipework and a return pump.
Since it is not part of the main tank, it will not effect your puffer - apart from improving his health in the long term.
Ecosystem sell ready made refugiums with everything you need - however this is more expensive than a diy version, and you still have to work out how water will move.
I think they also do a hang on back type, but it is only suitable for smaller tanks - up to 60 gallons (us).
Ecosystem Website
I suggest you have a thorough read of the above website - also Navarre is a good source of information on refugiums.
One other point - technically a refugium should be above the tank so that all the nice bugs can flow nicely into the tank without being ripped to pieces by the impeller. However this is an area of disagreement, and wont really matter for nitrate reduction.
- with regard to your nitrates, really it should not be possible for you to change 40% of 0ppm water in your tank with the high nitrate water that is there, without getting a reduction in the levels at least for a while.
Try getting 200ml of tank water in a jug, and adding 150ml of "new" salt water to it and measure the nitrate before and after - if there is really no change there is something very odd going on.
Also what buffer are you adding? I remember reading that there is something that can cause high nitrate readings.