secret of keeping blastomussa?????

[mandarin man]

Hi guys,

I have two colonies of blastomussas, both are dying. One head recedes one at a time and then move on to the next one. My friend's blasto does the same thing dying slowly

My water parameters are good:

ca-420
mg-1350
kh -8
Potassium - 350
ph-8.2
supplement -pol xtra, trace element, coral snow.
nitrate- .5
t5 for my light, my friend (MH) - I do not think it is the light issue.

I don't not know what is going. Why blasto is so hard to keep? Do you have any ideas any secret? The only thing I can think of is the water is too clean....

I also notice that LFS has blastos that have been in their tank for so long are also dying.

Please help