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I have a constant PH probe measuring PH of my fish tank water and I've noticed a spike in PH.

I've noticed at 7am (around the time the sun is coming up) PH rises from a baseline of 7.0 to 7.5 until about 11am when it settles back down toward 7.0.  Then at 1pm PH rises quickly to 8.0 and then crashes back down to 7.0 by 3pm.

The system is 18 months old and mature.  My tank is partially shaded and there's no major algae growth in the tank.

I'm not having any other issues but I wanted to better understand what's driving the PH.

Anyone have any experience with this? 

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Generally, photosynthesis will push pH upwards as carbonic acid (dissolved CO2) is consumed...again, having the result of driving pH upwards...

Keep in mind your fish and bacteria are also engaged in biological procces' of aerobic respiration while converting carbohydrates into energy (so in a sense, that is the opposite of photosynthesis) which produces CO2/carbonic acid, thereby driving the pH down...

It's what is called "diurnal pH fluctuation" and is a very common phenomenon...not only in AP systems, but in any 'ol freshwater lake, or salt water short, in any aquatic eco-system.

The scenario you described is just a little bit 'off' from the standard modus operandi...but by no means unexplainable... I bet if you were to give us more information like system water temps at different points in the day, feeding schedules, bio-mass ratios (plant, fish...bacterial bio-mass would be the tough one to quantify) etc...they would coincide with predictable patterns of biological activity that would effect pH swings...i.e. O2 consumption, CO2 production/consumption blablabla... 

Here are some neat problems/experiments from Cornell University meant to demonstrate such phenomena to students (originally written by William Carlsen from Pennsylvania State University)...

Hope this helped some...

Outstanding response, thank you Vlad.  Amazing how you learn so many new things from AP.

My system is inside a greenhouse so temps stay between 70F and 75F.  I do heat the water at night to keep it at 70F.  I feed small amounts throughout the day with an auto feeder and I have 20 5" Tilapia in the system right now.  My PH probes may be a little out of calibration but they're within a few % of accurate.  It's interesting to see the PH in the sump tanks higher than the fish tank.  It seems the water is running through the grow beds which is increasing the PH slightly.  The system is an IBC about 500 gallons total water.

I use RO water so my overall KH is very low while my GH is quite high 500 or so the last time I tested it.  I've buffered with potassium bicarb in the past as well as crushed coral and egg shells. 

I plan to test my KH and GH this weekend, I may need to buffer it up some.

The general trend is usually pH falling towards the evening and night...then rising in the mornings during the day...generally.

Your system appears to be 'double dipping'...which is a little odd...But I'm guessing that may be due to water temps warming up towards 1:00pm, driving out a bit of both O2 and CO2 (and having the net effect of driving a temporary pH spike). That coupled with the fact that around the same time, the sun is at it's peak this time of year, and your plants would be engaged in "full on" photosynthesis usurping 'significant' amounts of CO2 (this too would help drive the pH spike along with the warmer water temps...warmer water can hold not only less O2, but less CO2 as well)...

The dip around 11:00 I'd guess that perhaps your fish are becoming more active...feeding and moving round more...producing more CO2 (lowering pH)...which again the plants 'consume' (drives pH back up)? Don't take that last part as gospel or anything...It's just a somewhat logical/intuitive 'guess' from a distance...

It does though make good sense that the water in your sump generally has a higher pH than in your FT since there is likely more dissolved CO2 in your FT than in your sump. Being that fish are aerobically respiring and all. While the water falling into your sump has already been somewhat depleted of CO2 by first, your plants, then by the action of falling through the air and splashing into your sump tank. CO2 is only soluble in water under (albeit even slight) pressure. It would in essence be "off gassing" itself as it streamed through the air and fell into your sump. I noticed these same things in my big system. It's fun to go about figuring out why you are observing what you are observing 

A cheapie Arduino (or any PLC) and some in line water temp sensors (Dallas makes a decent waterproof temp's basically their DS18, only encased in stainless steel..DS18B20...cost under $10) would allow you to correlate in (almost) real-time your pH and temperature values...It's likely you would see a pattern emerge.

Yeah, the lower your KH, the more affected your water will be to such daily swings...though I'd probably let the night-time low pH fall to about 6.2 before I buffered...but that's just me.

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