pH enigma


The mad reefer
I seem to have an interesting occurrence ââ"šÂ¬Ã¢â‚¬Å“ not a problem really ââ"šÂ¬Ã¢â‚¬Å“ and I know that it is not a pH monitor calibration error as I recently calibrated my Pinpoint pH monitor.

Just before the actinic lights turn on, over my main tank, all parameters are normal:
pH 7.95 + or - 0.03
alkalinity 2.75 ââ"šÂ¬Ã¢â‚¬Å“ 3.00 meq/l
calcium 380 ââ"šÂ¬Ã¢â‚¬Å“ 400 1.024
NO3 0.0 ââ"šÂ¬Ã¢â‚¬Å“ 0.2
PO4 0
Temp 80 ââ"šÂ¬Ã¢â‚¬Å“ 81 degrees F

but after the actinics come on, over my 37 g reef tank, the pH drops to 7.85 + or - .03

Typical repeat chemistry values, 5 hrs after the MH lamp turns on, are the same as above but with a pH in the range of 8.02 + or ââ"šÂ¬Ã¢â‚¬Å“ 0.03 Just before the MH switches off, after an 8 hour photo period, the pH will max out at around 8.05. The actinics stay on an additional 3 hrs, during which time there is a slow drop in pH back to about 8.0

My question is, why does the pH drop after the actinics come on? As I understand it, the pH should rise when the lights come on. I ought to note that the pH is stabile for about two hours prior to the time the actinics come on. Perhaps I am seeing utilization of alkalinity and calcium - during the initial phase of the light cycle ââ"šÂ¬Ã¢â‚¬Å“ at a rate that exceeds CO2 production from respiration.

Since the pH and the Alkalinity are at the lower range of normal, maybe I should up the dose of overnight of additives, or add more limewater to my top off tank. Any comments about that?

I do not have a calcium reactor. I dose two part balanced additive solutions (Hardness Plus and Alkalinity Plus from NatuReef) overnightââ"šÂ¬Ã‚¦ but the last dose is 3 hours before the actinics turn on. I add kalkwasser to the RO water in my top-off tank, but it is a rather dilute solution and I do not know what the concentration is, nor do I know how much of that solution is added to my main tank as the evaporation rate varies daily. I use both activated carbon and RowaPhos. My salt mix is from Oceanic, and I do 5% water changes every 2 weeks. The weather in SW Florida is ideal this time of year and so the sliders (glass doors) are always wide open and I doubt there is a measurable difference between the CO2 in air inside and outside of the houseââ"šÂ¬Ã‚¦ and the reef aquarium is only 2 feet from a slider. Finally, the lights over my 10 g sump/refugium are on except between the hours of midnight and 3 AM

TIA... Larry
My only answer is that for some reason the ON of the actincs is causing the plants to release more CO2, or leaching out chemicals, thus lowering the pH
Thanks Boomer.

How'd you handle the chemistry if this was your tank - given the choice of 1) upping the two part additives, 2) increasing limewater, or 3) leaving things as is?

(BTW, I am still laughing about your years ago description of not wanting the chain of poop eaters in a DSB system.)
Two possibilities that I can think of:

1. The pH has nothing to do with the actinics, and is just dropping the whole time between mh lights off and mh lights on. That seems to fit pretty well, doesn't it? It doesn't necessarily need to be a constant steady drop.

2. Pinpoint monitors are impacted by electrical interference by certain lights. Perhaps your actinics are interfering with it:

Limewater and more aeration with fresh air would be good ways to raise the pH (which I would do). This article has more:

Low pH: Causes and Cures
Another possibility is when you are around. People in the room will exhale CO2, and potentially lower pH. So if you or your family showed up around the tank at about 8 AM, that might also contribute to a pH drop.

Also, I wouldn't expect much photosynthesis from actinics alone.
Hi Randy,

I agree that it is most likely a continuation of the pH drop after the last dose of 2 part additives. The pCO2 in the ambient air is not a factor for reasons already stated... and I am usually the only one in the room with the tank (which is next to a fully open 6 foot slider anyway).

I did read read all of your articles and decided to increase the concentration of limewater in my top off tank. I'll watch it for a few weeks and see what happens. - I should have mentioned that, visually, the tank is doing exceeding well... all SPS are growing nicely and polyp extension is normal.

Thanks... Larry
visually, the tank is doing exceeding well... all SPS are growing nicely and polyp extension is normal.

Sounds great.

You're welcome and happy reefing. :)