How to calculate Alk spike when adding volume

[TonyGio]

I need help with some math that’s above my mental capacity. I have 230 gallon system (120 DT, 70 FT, 40 sump) I keep my alk at 8.0 and use IO reef crystals. I do weekly 10% water changes which really doesn’t affect my alk much. I’m about to replace my 70 gallon frag tank with a 130 gallon frag tank. This will up my total water volume by 60 gallons. I’m concerned about the alk spike I’ll get with adding 60-80 gallons of reef crystals at a DKH of around 10. Does anyone know the math on this? I’ve been considering switching to TM to more closely match my ALK. Just hasn’t really been an issue until now. Any help is appreciated.

 

[Randy Holmes-Farley Verified Community Expert]

Combining 130 gallons water at 10 dKH with 160 gallons at 8 dKH will give 290 gallons of water at 8.9 dKH.

You can lower the alk in the new salt water before use if you want.