Can GFO bring alkalinity down?

[Jrodinnola]

I'm having a problem keeping the alkalinity in my tank up, and someone mentioned that running GFO can hurt my alk levels. However, I'm dosing alkalinity more than 3 times the amount of calcium, which seems pretty excessive. I'm using red sea supplements, and my apex doses 14ml per day of alk and 4ml of calcium via BRS dosers.. calcium usually stays right at 420. I'm running bulk reef supplies reccommended dosage of GFO for my size tank. Everything is semi-stable for the moment, but I want to get this figured out before I have a catastrophe on my hands because of any shenanigans I may be overlooking...

Sidenote: I use red sea coral pro salt for 25% water changes bi-weekly and supplement with red sea reef foundation supplements (Ca, KH, and Mg). The tank is a 30 mixed reef that has been running for 1.5yrs with mostly sps and few lps and a few softies.

ca- 420ppm

mg-1300ppm

alk- 7.5dkh

pH- 7.8-8.0

po4-0ppm

no3-3ppm

no2-0ppm

nh4-0ppm

salinity @1.026


-Jeremy

 

[ChadRaay86]

Short answer, yes. Most of the phosphate removers take alkalinity as well. A good rule of thumb is to expect higher alkalinity depletion according to the effectiveness of the product being used. ROWA phos and lanthanum chloride can deplete all very quickly, while GFO and other less harsh removers tend to leave it be a little better.

 

[kiko81]

Hi Jeremy. When you say few lps and few softies and sps, I would be imagining Its not alot of corals to be "eating" up your alk. I used GFO before and my alk really never drastically drop. I now use biopellet and everything has stayed pretty much stable. Just normal dosing for me.

 

[outssider]

I use brs high capacity gfo at their recommended amounts and am using more calcium than alkalinity

 

[Randy Holmes-Farley]

There are two ways GFO could reduce alk, but both are minor, IMO, and both also use calcium in a proportionate amount.

Reduced phosphate might increase calcification by corals and such, and that increased rate reduces alk (and calcium, although that may be too small to detect).

Also, there can be increased precipitation of calcium carbonate on and downstream of the GFO, possibly due to free iron atoms nucleating calcium carbonate.

Some forms of GFO may come in acid rinsed, and those may take up a small amount of alk directly and instantly and then stop, and base washed GFO might release some alk (also instantly and then stop).

 

[Jrodinnola]

To clarify, I use regular BRS GFO. Also, I have mostly sps.. about twenty 1"-2" sps frags, two softball sized monti caps, a 3" fungia plate and a 4" frogspawn along with some misc softies.

Randy- yesterday I did notice a white crusty substance which I assumed to be precipitated CaCO3 in my gfo reactor.. I scraped it off during my routine maintenance.. but I've only noticed it once since this has been going on. Also, if free iron ions are nucleating CaCO3, wouldn't calcium be used in a proportionate amount to carbonate? Because currently, the alk usage is 8 times higher than calcium usage (16ml KH to 2ml of Ca).. I would think it would be somewhat closer? Also, since i'm not sure if mL's are a good comparison point..I calculated the ppm vs dKH concentration usage to offer another view; The tank uses 2.24 dKH of alk and 5.6ppm of Ca daily. Maybe that helps? lol

 

[bertoni]

Calcium and carbonate are consumed at the rate of 2.8 dKH per 20 ppm calcium to form calcium carbonate. That's probably the rate at which your tank is consuming them, as well, but water changes and testing limitations can skew the supplementation rate, at least over the short haul.

 

[Randy Holmes-Farley]

I agree.

The ratio you are experiencing, if it continues for extended periods, is likely due to the salt mix used for water changes not exactly matching the tank, but alk can also be depleted in a few other (usually small) ways that aren't that noticeable unless calcification is fairly low (like in a FO or FOLR tank). I discuss them here:

When Do Calcium and Alkalinity Demand Not Exactly Balance?
http://reefkeeping.com/issues/2004-12/rhf/index.htm

 

[Jrodinnola]

I agree.

The ratio you are experiencing, if it continues for extended periods, is likely due to the salt mix used for water changes not exactly matching the tank, but alk can also be depleted in a few other (usually small) ways that aren't that noticeable unless calcification is fairly low (like in a FO or FOLR tank). I discuss them here:

When Do Calcium and Alkalinity Demand Not Exactly Balance?
http://reefkeeping.com/issues/2004-12/rhf/index.htm

Noted. I'll make and test a fresh batch of salt to see what the alkalinity is, but on the container of coral pro it claims to be 12dKH when the salinity is adjusted to 1.026 at 78°F. which makes me think its not the salt mix, because whenever I do a water change it boosts the dKH by about 1 or so dKH.

Also, Your article says- "After two days, many aquarists might conclude that they need additional alkalinity, when in reality, they need more of both calcium and alkalinity to stabilize the system."

So, according to your article, a good starting point to solve this problem is up my daily calcium dosage to the same as my alkalinity and observe changes over the next week?

 

[bertoni]

It might take more than a week for any drift to show up. I ended up testing calcium every month or so, and seldom did any adjustment at all after I started using a 2-part.

 

[tmz]

There is another way total alkalinity ,which is the akalinity test kits measure, is reduced, albeit very slightly . I learned it from i one of Randy's articles, btw;may not be worth mentioning in this thread but in case it comes up; PO4 is alkalinity ;reducing it reduces total alkalinity . Typically this would be insignificant since even at a very high PO4 level of 1ppm ,it's less than 1% of a total alkainity at 7 dkh(125ppm).