One For The Chemists

[CageUK]

My PH has mysteriously dropped recently.

I have changed a couple of things and I wanted to run this by the gurus to see if anything rang a bell.

I'm assuming that my problem is Co2 based although I haven't done the water aeration test to verify this. I have however tested the oxygen content of the water and it falls between 5 and 8 mg/l which is the normal level for salt water for my temperature (according to the test literature).

My other levels are:

PH 7.5-8.0(ish)
Calcium = 415ppm (dosing to raise)
Magnesium = 1020ppm (dosing to raise)
KH = 10dKH recently dosed up from 7dKH

My nitrates are still quite high @ 20ppm so I'm still struggling with them (so much for Polyfilter ).

I'm using Tropic marin triple buffer but this builds KH as well as PH so I'm reluctant to dose any more of this as my KH is already on the higher side of the scale. I also use Waterlife 8.3 buffer, but there is no indication as to wether this contains KH buffers.

I could previously get my PH to 8.3 with buffering relatively easily but now I'm struggling to get my PH above 8 and my night time readings are going as low as 7.5.
I have recently re-calibrated my digital PH monitor and checked this reading against a Tetratest liquid test kit and an API liquid test kit and the results were fairly consistent. The new reading was a little lower than the previous calibration but not by the difference that I'm seeing.

I have recently started turning off my skimmer for periods due to putting phyto into the tank to feed the Scallop. I thought that this may have contributed but the readings don't significantly change when I'm running the skimmer.
I'm still running an airstone in the filter chamber as I always have and I have my powerhead pointing to the surface giving me a reasonable disturbance. These items haven't changed.

The other change recently is that I'm occasionally feeding live food, namely Brine Shrimp and glass worms. This is normally 3 days a week (they do a deal at my lfs).

Any pearls of wisdom or enlightenment would be very much appreciated but go easy on me please, I haven't yet got my Phd in chemistry

Cheers

 

[Pleccy]

Does your tank have glass covers or a hood? The reason I ask this is because covers can restrict gas exchange, which could be causing a build up of CO2. Also the pH will rise and fall naturally over the 24hr period, however this can be prevented by adding Kalkwasser.

 

[CageUK]

Yes Pleccy. It's an Orca all in one type tank. I'm just mystified as to why my ph has dropped recently whereas it was ok previously. I have always experienced the night time drops but the range was 7.9-8.2 which I was much more comfortable with. The calibration would account for about .05 but this doesn't explain why it's now dropping to 7.5 at night.

If I'm right about my understanding of Kalkwasser, it will raise my KH which is ok at the moment.

The other thing I did think of, was that my GSP is now getting quite large. As it is photosynthetic it presumably makes quite a big contribution to my night time Co2. If that was the case though then surely the reverse would be true in the daytime and I would see a much larger swing in the daily PH.

 

[SkiFletch]

Few thoughts here Cage. First, have you added any new fish recently? The extra respiration from a new fish could be the cause of excess CO2.

Have you started closing the windows in your house/flat more now that the colder weather is here as opposed to before? When we close up our windows either for the heat of the summer to use A/C, or the cold of the winter to heat the place, we increase ambient CO2 in room air. This increase in ambient CO2 can take some time to manifest itself in lower pH for the aquarium but will catch up with you eventually.

Also, your Magnesium is very low. Low magnesium will drive the equillibrium reaction of calcium and carbonate to form calcium carbonate salts more towards the salt end. This will in turn lead to a slightly lower pH. When magnesium is high, the formation of calcium carbonate is inhibited leaving more carbonate (and especially bicarbonate) free. Furthermore, Bicarbonate is more favored to be used up when magnesium is low. This compounds the pH problem because bicarbonate leans pH towards higher levels and with it being used up first, pH seems lower while total alkalinity (which measures both carbonate and bicarbonate total) remains measured high.

Likely you have a combination of circumstances leading towards your low pH. The solution here IMO is to raise your magnesium back up and to not panick. Low pH isn't necessarily a bad thing. It's not the greatest for hard corals, but unless you have a tank full of sensitive acros, you should be fine.

 

[CageUK]

That's reassuring to hear Ski

I've not had a new fish but it hadn't clicked that now the windows are closed and we are running the 'oil fired' boiler more often and for longer it is bound to result in an increase of ambient Co2.

I'm dosing magnesium at the moment but it's taking ages to get the levels up.

I've only got a hammer and a solitary candy cane and both of them seem to be fine at the moment.

I'll continue the dosing and monitor the situation.

Cheers