Tang cichlid fin problems

Hi guys
Need some advice please
I 've got a Tang tank
70 gal
1600 liter external can filter and 2 sponge filters. Wave maker 3000lt periodically.
pool filter sand
2 x neolamp bricardi
2 x neolamp pulcher daffodill
3 x caudapunc
4 x leleupi
The neolamps and the caudapuncs trimmed each others fins severely no problem with the leleupi's . The fins don't seem to grow back and developed black edges. Al water readings perfect. Ph 8.5, am nil, nitrite nil and nitrate below 5. Water 20 % weekly changed with prepared water containing Safe and Tang salts . Help?

 

@Zirk not sure what you need help with...? the fins? Seems like there is an underlying problem with the mix of fish and/or territory problems causing the agression in the first place, so treating the fin problem will be a temporary solution? @Jack Stone will have some valuable input here, I am too new to Tangs to give any advice...

 

Thanks Francoisve
Each have their territories
Worried about the black edged fins, afraid that there might be something wrong with my water as I have to ad a lot of buffer to get the Ca and Gh up as the water has a ph of 8.5 but practically no Ca or Gh
Upped it with Tang buffer and epson salts before adding the fish and water changes is done with pre-prepared water.
Al water tests are perfect

 

My tank

 

if the water is within parametes you may need to look elsewhere. nutrition? most fish recover quickly from a nipped or torn fin,unless off course they are not healthy... what are you feeding? water temp?

 

Water temp 27degrees
food variety of brands from micro pellets to flake to frozen mysis shrimp
The seem to enjoy them all

 

All sound good... No ideas further from me Have you tried a general tonic or anti - fungus treatment? Garlic should work well...