Water PH changes and light

Can someone explain why ph changes when lights are on or off? I remember always being told to check the water's ph at the same time each day to ensure accuracy, and of course part of the reason to keep a fuge light on 24/7 is that it causes less ph swing, since the water is always exposed to light.

Or am I understanding this wrong?

Photosynthetic critters (algae, corals, trees, grass, etc.) take in carbon dioxide (CO2) and release oxygen. When carbon dioxide mixes with water, it creates carbonic acid, which drops pH. Photosynthesis primarily happens in the light (it's a process fueled by light). So when photosynthesis is happening (when it's light out), CO2 is constantly being taken up by these organisms, so it's not getting a chance to sit in the water and create carbonic acid.

At night time, most of the photosynthesis shuts down, so CO2 accumulates in the water, forming carbonic acid, and dropping the pH (making the water more acidic).

This is why the pH is lowest first thing in the morning, after the lights have been off for a long time. And this is why pH tends to be highest at the end of the day, after the lights have been on and organisms have been photosynthesizing all day long.

If you run your fuge 24/7 or opposite the tank lights, it allows whatever is in your fuge to continue photosynthesizing during that time. So there will always be something in your system taking up carbon dioxide, and preventing the pH from dropping. Running a refugium on an opposite or 24/7 light schedule is one of the easiest and most effective ways to prevent pH swings.

Awesome!!! Thanks for the explanation! Makes sense to me :) :bowdown:


I was just wondering cuz I'm curing some garf rocks, outdoors, waiting for the day when the PH doesn't go up anymore for it to be safe to go into the tank. So I fill it, and the next day, I check to see if the PH went up, and it's still going up :frustrat::frustrat::frustrat:

So I'll dump out the water, and fill it up with more water, and repeat the process. Which then got me thinking maybe I"m going about this all wrong....maybe it has to do with the time I check it. But since there is no algae or organisms in the container to cause the PH upswing, I guess it really is just my rock that's causing it to go up.

I'm going to make more garf rock with more salt this time, and cure it outside. The first ones are not really failed, but just taken a long time to cure (I ended up curing indoors since it turned winter), then it sat dry for a few months while I decided what to do with them. So I'm making a new batch -- a lot more, to make up for the lack-of-rocks in my 125g. I just have to be more diligent with changing out my water with this next batch.

This is also why you may notice if you have a big party at your house, and you have a lot of people crowded around your fish tank, and the windows are closed, your tank's pH can drop. Because of all the CO2 the people in the room are creating.

Bifferwine said:

This is also why you may notice if you have a big party at your house, and you have a lot of people crowded around your fish tank, and the windows are closed, your tank's pH can drop. Because of all the CO2 the people in the room are creating.

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WOoooooow....never thought of that! :shock:

Hehehe...and I had 8 girls in my basement for a slumber party last weekend too! LOL