Help fighting finrot for 2 months, PLEASE!

[BaileyB]

I have a small porcupine puffer I got a couple months back. He was in QT for over 40 days with copper safe at 1.5ppm, also treated with Maracyn 2, furan 2, and prazi pro. He was added to a cycled 145 gallon tank with new rock and sand. Had VERY SLOWLY started developing tail/fin rot. It’ll get bad then get better. Then one day look back and it’s worse again. Weekly-bi weekly water changes done but all the fish in the tank are still very small so don’t have a huge bioload so don’t think it’s due to water quality? I recently got seachem metroplex, kanaplex, and focus and have been soaking his frozen food in the medication for about 20min to an hour before I feed, been feeding for about a week and it seems like it’s getting worse again. Who can help me?! I’ve spent about $150 on medications to try and get rid of this one simple infection but can’t seem to get rid of it. Any help?

 

[BaileyB]

Here’s my puffer and his tail, don’t mind his big belly he just ate

 

[BaileyB]

Bump?

 

[Uncle99]

It’s interesting that only the tail shows some damage, the others look good.
Any possibility that someone nips at the tail?

 

[Jay Hemdal]

I agree with @Uncle99 - the jagged nature of the fin and the coming and going all strike me as being caused by another fish. What other fish are in with it? Any aggressive inverts that could be tagging it at night?

Jay

p.s. - just a side note, Coppersafe at 1.5 ppm is too low to control most protozoans. I use at least 2 ppm. A full dose is more like 2.25 ppm