Your alk is low because your trying to keep a reef tank. Dosing and testing are part of owning a reef. When the time comes that your reef is consuming more than water changes can keep up with you have to dose, that is just the way it is. The coraline in your photos looks like its doing pretty good. It will be consuming your CA MG and alk so dose. Its time to get into testing, you should be testing CA MG and Alk and dosing all three accordingly. Test your parameters and use the calculator to dose the proper amounts. What you use to dose is up to you but the DIY three parts are good for beginners. Yes baking soda is the thing to use for alk. You can use lime to dose ca and alk at the same time but it has to be dripped and it doesnt sound like you understand reef chemistry enough to be using lime. Do you have an ato? Its time to start reading up on reef chemistry, in the mean time do some bigger water changes. In reality you should be balancing your water changes which means pre balancing prior to use anyways. This means dosing the water change water with anything that it is lacking prior to use. For example IO needs CA and MG to be in balance before even using. Start reading!
Don
