seachem marine ph buffer any good??

yes

but you will see variations between night and day as Xmetalfan explained above

at night algae and any other plant matter, consumes oxygen and exhumes CO2
CO2 drives down ph

daytime the same plants and algae consume CO2 and exhume oxygen - which means CO2 levels fall and PH increases

you get slight variations - maybe 7.8 if you tested at night and 8.2 - 8.3 during day
and this is normal

so test at same time - late afternoon each day
and dont add buffers based on readings taken when lights are not on
Steve

 

but will changes from 7.8,8.2, 8.3 stress or kill the fish?

 

No, everything will be fine.

 

I am using it to increase the alakanility and I dosed from 5 to about 8 without causing significant drop in Calcium so I think it's good. By the way my PH is always staying at 8.2 even before dosing when tested with Salifert kit.

 

Alkalinity controls pH for the most part. You can have the same pH and have a changing alkalinity all the time. I really don't want to go into all the details of how it works but it does.