pH and Alkalinity (KH
, carbonate hardness) are related, as is pH and light (via photosynthesis occurence). You pH will tend to swing down throughout the lights-off cycle, then will swing back up when the lights come on. The extent of this swing can partially be 'controlled' by maintaining a proper Alkalinity level. Most advice I've seen is to maintain it between 7 and 12 dKH
, I've seen some advice saying 8-14. If it gets too high, depending on your Calcium level, you can start to get precipitate in your sump and onto your equipment. I have never experienced this, and have kept my tanks anywhere from 7.0 to 11.5 dKH
(using Salifert test kit), and the only reef tank (the only place where you would have a lot of Calcium) I keep the CA
around 430 currently and the KH
I try to maintain it around 8-9.
If you're cycling the tank, you are probably going to see these parameters vary wildly. One thing that I noticed was that when I tore down a tank and temporarily kept the LR
in a tub full of water (FW
) that when I set the tank back up (it was a 200 w/150 lbs LR
) the KH
would drop a full point each day. Eventually the rock sucked up all the carbonate it 'needed' and I haven't had to adjust the KH
at all for months.
I use Brightwell Aquatics Alkalin-8.3P to boost the KH
. That product touts that it will raise your pH to 8.3 and then you can continue to add it to boost your KH
without affecting pH. What I've basically found is that while it can raise your KH
well (although at about 1/2 the recommended dose) usually your KH
stabilizes out to the point where you don't need to adjust it that often, and your pH will continue to drop.
Kind of a long answer but I hope it helps.
Also I have found the use of a pH meter to be COMPLETELY indispensable. I bought the Hanna Instruments HI 98129 combo pH / TDS
meter and will never go back. Only drawback is that it costs like $150 new and then you need to buy the calibration and storage solutions. I got a sweet deal on one on eBay with all of that for $100 though. Well worth every penny.