using sodium carbonate anhydrous for KH

can i use sodium carbonate anhydrous instead of baking soda for kh ph?

 

Yes you can, it will cause a short term pH jump instead of decrease like baking soda. It will support your pH slightly better than baking soda also.

 

dosing

should i dose the crystals or dilute in water then dose?

 

Always dilute before using. Slowly add to the tank in a high flow area. Add in the sump if at all possible.

 

is dosing powder directly bad or its ok? any reason?

 

It is bad, you have chemical reactions that occur so you have heat and ammonia (IIRR) created. You want to mix up a batch and let it settle for an hour or so before adding to the tank.