ph in my reef

[crystalreef_aquarium]

What causes ph to swing from dark to when the light are on?
Why I ask, is my coral does fine for most of the day and then after about 6 hours, it starts to close up. Ph is 8.4 when I check it at night and then, it goes up to 8.6-8.7 when the light have been on for a while. My Kh is around 10. Phosphate 0, nitrate 0, amonia 0.
I use a ro/di unit that is only about 1 month old, I do not have a TDS meter, but I do not have a problem with any unwanted alge, and I only have two small clown fish in there and I only feed them every other day. My 175w MH bulb is old though and I am just waiting to get a new one. My skimmer is, from what I can see working ok but it seems to work better when the lights have been off for the night, I have good water movement in the tank as well.
The tank is a 75 gallon Dutch.
The corals that I have are Green hammer,devils hand leather and pulsing xenia not sure the type, it is doing great it has doubled in the last 2-3 weeks but the rest of the corals not so happy.
Sorry for typing so much but I thought that I would give as much info as I could.

 

[Anonymous]

Welcome to RDO, Crystalreef!

First to answer your pH question. pH will often dip a bit at night because of rises in CO2 largely because algae stop consuming it and producing O2. You can minimize this effect by either starting a refugium with the lights on at night, or (and you should do this anyway) agitating the water surface by pointing a powerhead at it.

However, I doubt that is the reason your corals are closing. You will notice that corals have a sense of schedule. They get used to the time of day the lights go on, and the time they go off. As nighttime approaches, they start to close up. No biggie. Also I have noticed that they respond to changes of season as well- now that the days are getting shorter, they start closing up earlier.

Sounds pretty normal to me.

Although one thing I would suggest- one 175 watt bulb isn't a heck of a lot for a 75 gallon. You might want to think about getting one more, or supplementing with VHOs to increase your light. That may also help.

Hope that helps!

HD