pH Stabilization Problem

[cusemarine]

For the last week the pH in my main tank has been fluctuating between 7.8 in the morning and 8.1-8.2 in the evening. I usually check pH monthly since I've never had issues before at keeping it at 8.4. Alkalinity is 4.75, and carbonate hardness is 11 dKH. I'd like to discuss (1) what is causing the pH instability and (2) what can be done to stabilize the pH at 8.2-8.4 while keeping optimum alkalinity and dKH levels?

I initially tried Kent Superbuffer but the dKH remains consistent at 11 and I don't want to overdose the tank with carbonate. The invertes and fish seem unaffected and continue to eat but I'm concerned that longterm pH instability will cause harm. My tank specs are included below. Thanks

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90-gallon marine tank with a couple clowns and sailfin tang. Several invertes including 12 emerald green crabs, 20 hermit crabs, 3 peppermint shrimp, 6 turbo snails, and a BTA. Protein skimmer with 100-pounds live rock, Total 495 watts lighting w/2-MH, power compacts and lunars. Ammonia & Nitrite = 0, Nitrate = 10, specific gravity: 1.025. Using Sea-Lab 28 to maintain calcium at 400 ppm.

 

[malkore]

am and pm fluctuation is normal. at night, CO2 builds up and lowers the pH. during the day, it gets removed by algaes, and the pH rises.

however, a .4 swing is a bit much for this 'natural' occurance. I typically see a .2 change in my tank at the most.

Alkalinity IS hardness...what brand test kit are you using? 4.75meq/l is a bit high, but 11dKh is good...so one of your kits is wrong - because at 11dKh you should be 3.9meq/l

if it's 4.75meq/l then dKh is 13.3


what do you have for pumps/powerheads? maybe you're lacking enough current?

 

[cusemarine]

I'm using the fasTest/Seatest for alkalinity which has been v.difficult to read. The water color change is subtle at best. I can't remember the name of the dKH test kit but it is v.easy to read and I put my faith in that one (indicates 11 dKH).

I have not considered water current. I rely primarily on my sump return pump for water movement. It is pumping at 900-950 gph (cycles the tank every 6 minutes) and is angled to provide some current in the tank. Can you explain how current could effect the pH?

 

[Jason]

PH shift from night to day.....

If you want it to stay more stable, get more live rock.

 

[malkore]

So you're barely at 10 x turnover, and that's assuming you didn't lose gph due to head clearance of the pump.

I'd add a MaxiJet 1200, at the top, and angle it so it ripples the surface of the water quite a bit. That's going to improve gas exchange, which should prevent CO2 buildup over night, and thus reduce pH fluctuation from that.

I too would not put faith into the fastest/seatest kit.
Could the dKh kit be Aquarium Pharm/Doc Welfish? Basically where each drop equals 1 dKh, and it starts off blue and turns yellow when you've hit the right dKh of the tank?
If so, I've compared that 'cheap' kit to my 'expensive' seachem alkalinity kit, and they give the same figures...so I'd say if you've got that kit, then your 11dKh is probably correct (and a good level)

 

[Jermz79]

I also notice a .2 swing in my Ph from night to day! Usually 8.2 at night 8.0 in the morning!

Malkore hit the nail on the head!