Gradually increasing PH - HELP!


New member
Hi everyone - I need to get to the root cause of my high PH issues. I understand how to lower my ph through reading a ton of stuff, but I'm hoping to understand why I have such an issue in the first place.

My tank has gradually been increasing ph over the last several weeks. I consistently do a 20% water change every 3 to 4 weeks. Generally speaking the water change does very little to reduce the ph- if at all and the ph grows 2 to 3 points every few days. Right now my tank is reading 8.61 during the day and 8.55 at night. I need help to figure out what's going on. I plan on adding some airstones in the sump to increase oxygenation to fix the problem - but would like input from you guys before I try anything.

Let me start with the biometrics of my tank:
I'm not sure what you need so I'll tell you everything:

300 gallon 72"x36"x27 Display Tank
Inline 75 gallon sump
Inline 29 gallon seahorse tank
350lbs of live rock
200lbs of live sand
Vertex Alpha 250 skimmer
8 48" T5HO lights (adding 4 Ecotech Radions tonight!)
4 MP40 powerheads
Mag18 return pump
No Fuge (just some live rock in one chamber of the sump)
36 watt UV Sterilzer running 24x7
3 dosing pumps for MG, CA and ALK
Neptune Apex controlling everything.
Not sure if you need my fish inventory - if so I can add it. Here's the current measurements on my tank using Red Sea and Elos test kits:

KH is 8
PH averages around 8.55 at night and 8.61 during the day.
MG is 1200
CA is 440

Any ideas?! Help please! Oh - i guess before i kill my fish - is there a general rule about how fast or slow you can lower PH without killing fish?


Still Learning
I will start with the most obvious one (you probably already have thought of), have you recently done a calibration test on your Ph probe? I do one at the beginning of every month as routine maintenance. How old is your probe?

Also check if your Vortechs are putting out as much. Maybe they need to be cleaned or something? I know mine slow down after a while.

See if you have been using a new salt mix or a new batch? Maybe this new batch is giving you the higher Ph values.

See if your Nitrate has gone up or not.

Hope this helps.



New member
Thanks Psyops - I have 2 PH probes, both recalibrated within the last 3 weeks. One is in the sump, and the other is in the overflow of the DT. They usually both read within 1 to 2 points of each other.

I definitely should try cleaning my Vortechs - they probably haven't had a good cleaning in a year. I have a ton of surface agitation though. It's like little waves crashing around at the surface of the water - are you thinking it's an issue with gas exchange because of lack of water movement?

I'll check my salt mix ph after i make a new batch. The only thing that makes me want to rule that out is that generally after a water change the PH goes down a few points and then over the course of weeks it gradually builds - this would indicate it's something in the tank creating the build in CO2.

Randy Holmes-Farley

Reef Chemist
Premium Member
I agree that unless you are using limewater, those pH values are likely in error. pH almost never gets this high without using high pH supplements. Also, it is not high enough to really hurt anything. :)


New member
I'm at a complete loss - I've wondered if there's a problem with my Apex that is causing the accuracy to slip. It seems unlikely that both probes would fail and still be a few points of each other.

If it is a problem with the Apex, it would make sense that both probes are reading high. I'll calibrate both probes again and see what happens. My instinct based on my last calibration is that it will still read high.

I guess the only other option is to get another independent PH meter to validate the results. I wish I could use drop tests just to see if i'm in the right ballpark, but I can't see slight differences in shades of certain colors - which is extremely frustrating when it comes to test kits in this hobby.

Ron Reefman

New member
Do you have a friend or local club member you can get to help. Borrow their test kit (and eyes) to double check your probe numbers? Being part of a local club can be so useful and helpful. I have 4 or 5 good size tanks and experienced reefers within a mile or two of our house. This kind of double check would be easy. Good luck.


New member
Thanks for all the advice. Here's an update; I've been having some small little issues with my Apex (non-responsive WXM module, lost outlets, etc) so I decided to do a firmware update to establish a good starting point. Mind you I was only 4 months behind from their current firmware version.

Anyway - after the update - I recalibrated both ph meters and was surprised to see the values. PH Probe 1 immediately climbed to 9.87 and PH Proble 2 went to 8.35. I did another calibration on PH1 - this time it went to 8.02. I swapped probe positions on the apex manually and Probe 1 (previously Probe2) was reading 8.35 and Probe 2 was reading 8.02.

Obviously a probe issue. I'm putting in an order for another probe today as well as a handheld ph meter (just for peace of mind). Thanks for everyone's advice.


Still Learning
Sorry for not replying sooner. Was out of town with work. As Randy said try to check the pH out of the water. I had this problem in the past.

I had the same exact pH issue with the Apex. It was not the probes. I could not figure out what it was but one day both meters showed about the same reading. I think I just turned off my Apex for a few minutes and turned it back on.

I still think it could be a ground loop issue as Randy said. I also cleaned up my wiring. I did those two things at the same time, so don't know which of the issues it was.

I used to have issues with the Mag18 in an older fish only system. Randy knows what they are called. Something to cancel out the current in the tank. If you have another pump, try it. Also watch your wiring on your lighting system. This was how my friend's tank was having problems.