pH too low, caused by topping off?

[leafyseadragon]

I have been cycling my tank for two weeks now. As the water has evaporated, I have been replacing it with fresh tap water (I know, I know-- I will start using RO from now on).

Today I checked the readings:

NH3 - 0
NO2 - .3 g/ml
ph - 8.0

My 37 gallon tank has 12 lbs of live rock, 32 lbs of base rock, and 40 lbs of aragonite sand in it. Nothing else in there except lots of diatoms.

I thought the aragonite sand would buffer the pH to something closer to 8.3, even though I am adding fresh water with a pH of 7.0. Should I buffer the fresh water before topping off the tank? Every reference I have read says top off the tank with fresh water, but I have never seen one where they say buffer the water first.

So, what should I do at this point? Add sodium bicarb or go get something else to add to the tank?

 

[reefrunner69]

Aragonite will not buffer above 8.0. If your tank is cycling, the ph is gonna be squirely, once the cyle is done, do a large water change and it should get things back on track for you.

 

[leafyseadragon]

Thanks for the advice, Kevin.

Now about topping off the tank in the future. Do you buffer your fresh water before pouring it in? It seems sensible, but no book I've read ever mentions this.

 

[reefrunner69]

The salt should provide plenty of buffer for the tank, I wouldn't buffer the FW unless you find a deficiency in your Salt Mix, and then I'd change salt mix first. The only thing that evaporates is the water, the buffering agents stay in the water.

 

[thorn]

also, tap and distilled waters are notoriously bad for holding a buffer (and as such, your stable pH), moving to R/O will help a great deal.