Marine Salt lower pH levels?

[fearkitten]

I'm running a 30 Gallon reef tank (been up and running for about 3.5 months now) and keep encountering pH levels that are lower than recommended/normal and I fear what long term effects this will have on my fish/corals. All other parameters in my tank have been testing at normal levels but my pH runs relatively at about 7.4 to 7.5 on any given day. Everything in my tank looks healthy and happy at least, but I obviously want to do my best to provide the best quality water chemistry I can for my stock in order for it to thrive well in the long run.

After performing hours of online research I tested a couple theories and still have no real answers.

First I took into account that gas exchange may have a negative effect on maintaining my pH levels so I measured the pH of my tank water (7.5 as usual) and removed a glass full and aerated it with an airstone for an hour, then remeasured the pH of the water in the glass and there was no change. This was not really all that surprising to me though as I have adequate water movement throughout the tank (377 GPH) and one of my power-heads is specifically positioned upward to break the water tension and remove the chance of poor gas exchange in my water (also no lid for better oxygen flow).

Second I read in multiple places that using liquid Amquel can lower pH quite drastically in certain water conditions. I usually was using it to treat my tap water before water changes so I thought perhaps this was the culprit. I measured the pH of my tap water right out of the sink and it read at 8.7 pH.
Then after adding proper amounts of marine salt and Amquel to a 5 gallon bucket of tap water, I remeasured and it dropped down to 7.5 as expected. So for a moment I was convinced it was the Amquel. I dumped that bucket of water and made a new batch, this time I treated the water with Prime first - which, when added on its own into the freshwater had no effect on my tap's pH levels as they still read at about 8.7 but as soon as I mixed in my marine salt it dropped back down to 7.5!

Is it possible that the marine salt is having this effect? And if so why/how and what can be done about it? Or am I just an incredibly dumb newbie who is missing something really obvious? Any clues as to why this is happening would be very much appreciated! And apologies in advanced if this is a dumb question...

[bertoni]

To Reef Central

What is the alkalinity in the tank?

I would measure the pH of some freshly-mixed saltwater after three hours of aeration with a powerhead in the container. Some salt mixes start at a low pH when first mixed. If the pH doesn't rise to the 8.0-8.3 range when mixed outdoors, then something's wrong with the pH kit.

I don't believe that the pH is at 7.4 in the tank. That by itself sounds like measurement error. You could try aerating a glass of tankwater outside for three hours, again with a powerhead or airstone. The pH should be in the same range as the freshly-mixed salt, assuming the alkalinity is okay.

[Randy Holmes-Farley]

What you showed with the aeration test was that either your home has excess CO2 in the air (very common) or the pH measurement is off (also common).

Aeration with outside air is another test, and it will rise if the alkalinity is above 6 dKH.

There is no need to search further. pH of aerated seawater is ONLY determined by the alkalinity and the CO2 level.

This has more:

Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm

[fearkitten]

Hello again, thanks for the kind welcome and replies...

Alkalinity: 10 dKH
Calcium: 420 mg/L

From past readings I believe these parameters are right/reasonable for my tank, correct?

Thanks for the suggestions! I will aerate some freshly mixed salt water out of doors as soon as I get a chance and get back to you on that - if there are still no changes I will try a different pH test. Currently I am using an Oakton Eco digital pH meter that is supposed to measure within a ±0.1 accuracy range so I would be really disappointed if that were the cause of the low pH results.

[fearkitten]

On a side note, I do feel it is unlikely I would have much excess CO2 in my home as we leave our doors and windows wide open through a majority of the day to freshen up our home and we also have many house plants which I would assume would help with reducing any CO2 that there is...

[fearkitten]

Oh and just to keep this thread updated, someone on another board mentioned that perhaps the brand of pH meter I am using is not giving accurate readings because it is not properly designed to be used with saltwater. It is specifically made for gardening, which may explain why it reads our freshwater accurately at 8.5-8.7 but drops immediately to 7.5 range when marine salt has been added. Stores are closed now but I will purchase a new test kit or meter at the LFS tomorrow and go from there. Thanks again everyone! ^_^

[Randy Holmes-Farley]

The plants won't help much, but the doors and windows sure will.

No, the meter type won't matter, as long as it was properly calibrated. How was it calibrated? Single point calibration may be inaccurate.

[fearkitten]

Yeah - we only shut our doors at night (and even then a window or two is still cracked) so I would be really surprised if excess CO2 in my house would be the cause of the low pH.

The meter is three-point calibration with auto-buffer recognition. I guess the reason why I thought it was related to the meter being unable to accurately read saltwater is because I otherwise do not understand why it would measure my water straight from the tap as 8.5-8.7 but as soon as I add marine salt to it immediately drops to 7.5... that just seems a little strange to me.

[Randy Holmes-Farley]

Tap water often has a high pH. Utilities raise it intentionally to reduce copper and lead corrosion release from pipes, but the salt mixes contain chemicals that then alter the pH and equilibrate with CO2 in the air.

[fearkitten]

It was indeed the pH meter that was throwing me off. I purchased a droplet test kit today and my water is now reading at a healthier 8.1 instead of 7.5. So, lesson learned here - gardening pH or any pH meters not intended specifically for saltwater use will not produce accurate results for saltwater pH. Thanks again for your help everyone!

[bertoni]

Okay, that pH makes more sense. pH measurement problems are very common.

[Randy Holmes-Farley]

Well, OK. I don't believe it was the nature of the meter, but perhaps the calibration. Anyway, I'm glad things appear fine.