pH as a water change indicator?

[CombatWombat]

I am about to upgrade the lower end pH controller on my girlfriend's CO2 injected planted tank with a nicer model, at which point I will find myself with a perfectly functional pH controller and no real use for it.. I figured I would just stick it into my 110G cichlid tank and read the pH. And then I thought, can I use this to signal a need to do a water change to myself? We know pH in a tank drops over time, so I can set it to light up a light bulb on my desk if it drops below a set point.

My question to you guys is, would it work? How much does pH drop between your typical water changes? What ΔpH should my water change alarm be set for, 0.1, 0.2? Or does it only start to drop when the tank is already pretty far gone?

 

[Hendre]

I think that is risky, as most water contains carbonate buffers that keep the water relatively stable, but once those run low the pH can quickly run amok. I may be wrong, duanes will probably know more

 

[twentyleagues]

Hendre is correct ph is controlled/effected by carbonate hardness. So will a pH controller alone tell you a water change is needed imo no. It isn't a "safe" way to dictate when to do a waterchange. If I waited for my pH to drop to change water I'd be waiting forever, my carbonate hardness is really high. Alternately if your carbonate hardness is low you could be doing daily waterchanges.

 

[BIG-G]

Another thing to note as the others have said a tank with water that has a higher buffering capacity will take longer for the Ph to drop and during this time the normal nitrogen cycle will still be working and producing nitrates.
So the nitrates in the tank could reach an unhealthy level well before the Ph starts to drop.

 

[neutrino]

Agree with everyone else, pH is not what you should base your normal water changes on. In fact, among the reasons for doing water changes is to keep water chemistry stable.

In some tanks pH might oscillate slightly during the course of a day and constant precise monitoring could reflect that, but there are few fish you'd keep in water conditions where your pH could drop beyond such slight oscillations by the time you'd do water changes. In most tanks, waiting for such a pH change would mean you're waiting for the water to go bad before doing a water change. Bad idea.

Most fish remain healthy in a wider pH range than some people understand and can handle changes in pH better than some people understand, but to keep your fish healthy and long lived you don't want pH bouncing up and down constantly.

 

[neutrino]

...Upon reflection, I should add the qualification, in view of the planted tank you mention, there are what some of us would consider specialized setups where pH might naturally fluctuate more than most tanks, and maybe that's the source of the question-- or you're coming from what would be an esoteric type of tank for the average fishkeeper. But in most setups a significant pH drop indicates a problem.

 

[duanes]

When a tank is first set up (but after i is cycled) watching pH fluctuations is one way to determine frequency of water changes.
An aquarist might think 1 water change per month is adequate, but if pH crashes from a tap water norm of 7.5 to a pH in the tank of 6, in a week, or less that's a red light.
But I agree with the others a couple of 10ths, even during morning to night is not unusual.
Because my latest tank is overly planted, early morning pH might test at 7.4 after plants have been shedding CO2 all night, but when I test pH at 6pm, pH might read 8 when the plants are visibly oozing O2 bubbles.
So waiting for a probe to determine a regular schedule is not realistic for an average freshwater aquarist, the way it might be for a reef keeper to add buffers.

 

[duanes]

As an after thought, when adding a bunch of new fish, or after fish has grown substantially, or some other radical change in an aquariums norm.
A series of pH tests might help determine the new more stringent water change schedule to maintain stability.
Because an 8" oscar (for example) will put out about triple the uric acid as a 3" juvie, or 6 more individuals in a tank may require an upgraded water change schedule,

 

[epond83]

I agree with everything said already, so many smart people on this site!

With that said it sounds like the pH probe is hooked up to a smart controller of some sort. I think it would be interesting if you could log the pH over time and maintain your current water change schedule noting them on the pH log. This along with your tap water parameters could give insight to your setup.

 

[CombatWombat]

Thanks guys, that did put things into perspective. Guess I will just keep it in the tank as a constant monitoring tool. My water is pretty soft, and there are only two measly anubiases for plants in there, surviving against all odds of oscar biting lol. That's why I thought I could detect a change. I think I'll log it manually (as I said, the thing's on the lower end. No logging, just monitoring and a power up for an outlet if above/below setpoint) along with NO3 and see if there's anything resembling a stable relationship up until 30-40 ppm NO3, at which point a water change is absolutely required. I won't get my hopes up though.

I was just enjoying the whole system I had set up for my GF's planted tank, CO2 feedback loop through pH controller, heater and chiller... Thought I could do something useful with the extra hardware for my big tank too.