Lower Alkalinity

[RichReef7]

I'm a relatively new at this. Trying to avoid expensive mistakes.
Cycled my tank for almost 3 months adding cleanup crew and a few fish. Life seemed good. Spend $200+ on frags and within days they were toast.
I’m pretty sure my Alk being 12 dkh fried them.
I need to get it lowered and have seen multiple threads about using lemon juice and the best way to drop it is with muratic acid.
I'm willing to do that but don't know what the mixture should be.
Can anyone tell me please.

 

[blaxsun]

Invest in a good alkalinity testing kit (Salifert, etc.)