mg/kh problem

[untamed]

I don't know how you are measuring pH, but if you were able to measure it every hour over 24 hours you would find that it rises/falls in a daily cycle. It might change by .5 or more...and that is very normal

So...your measured "change" between 7.8 and 8.0 might not really be a change at all and might just be related to the time of day you took the measurement. pH rises when your lights come on and falls when they go off.

I would also add that IMO, don't attempt to manipulate your pH with additives until you've got a lot more experience. Start by focussing your attention on Alk (KH) and Ca. In fact, that buffer you are adding is mostly about Alk. It's affect on pH would only be a side effect of it's additional Alk (crude summarization...).

However, rather than adding "buffer" and impacting your Alk...you should be using an additive system that is contributing both Ca and Alk. After that, changing Alk or Ca independently should be something you do relatively infrequently.

You are likely to be fine with Strontium, Iodine and trace elements which will be supplied in your water changes.