I've been given the link to your site from another forum I post on... Am glad to have been sent in your direction!
I have been keeping puffers in one form or another for several years, and for the last year had my heart set on a Mbu. Having worked out size and perameters etc, he was to go into my community tank until he got a little larger and then I was to custom build him something in the region of a 1000g tank.
Finally last wednesday one came into my -notoriously bad- LFS and I snapped him up...
He's 4iches not inc. tail.
Unfortunately not having a quarantine tank set up, and as they were being iffy about keeping him in for awhile, I bought him that afternoon and put him into my "Slow and Nice Tempered" community tank, and he is fine with all other inhabitants. The tank is well over a year mature, and I removed several fish to adjust for his bio-load. All my levels have stayed exactly the same, pH is 7, Ammonia 0, Nitrite 0mgl, and Nitrate 0mgl...
He has however got what looks like a small Bacterial infection on the rear section of his tail (As shown in the below picture, apologies for quality) which looks like a small lesion of some kind... with ever so slight ragged skin around the white wound

Does anyone have any suggestions as to what to treat this with? It's been there since I got him from the damn LFS, but I don't wanna take him back as there isn't any more, and I love 'im already
So far I've just been doing daily 20 percent water changes, while I found someone who might know about this kind of stuff... Doesn't seem to have spread, doesn't seem to have got worse...
If anyone has any suggestions, I'd be supremely grateful... Thank you for taking time out to read this post...
A most concerned AJ.
Owner of Olaf Leviathan Mbu Esq.