Can Freshwater top offs lower ph

It is aged FW? If it's straight out of the tap you could be seeing CO2 offgassing, although I don't think top-off quantities would effect your ph that much.

Edit- Re-read and saw that you're using RO, so that's not what it is.

i have gotten a very good explanation of this from a member named squakbert. (or something like that.)

Because RO water takes out most of the ions in the water, it really doesnt have a ph, and the ph of it is greatly altered by anything basic or acidic added into the water.

So it shouldnt be lowering you PH, because once it gets added into the high PH water, its ph will change drastically and quickly to match the tank.

I have had low ph problems before, you can simply buffer it... which is what i did for a while but after about 2 months i switched salt from regular instant ocean, to reef crystals. That cured the problem, my ph is now very consistent around 8.2-8.3.

well, i shouldnt say it doesnt have a ph, because it does. But there are no affective ways of reading the ph, and once its added into the tank, the ph of it will change very quickly, to whatever the ph in the tank is.

Top offs should not effect PH, unless the PH is read right before the water is added. All the alkaline elements and compounds should stay behind (just like the salt) as water evaporates.

You can buy PH buffer at the LFS and raise your Ph that way.