Carbon effects on PO4

TheDude · Jan 6, 2015

Hello fellow reefers,

I'm not sure this is in the right fourm but I'm gonna give it a go and if it gets moved then I completely understand.

First a little background. I am currently running a 2 little fishes reactor with Seachem pellets. I have noticed a considerable amount of brown slim on my rocks and attribute it to the pellets. I have lowered my flow ect and have finally come to the realization that I'm just not doing it right. So, I bought a dual stage GFO/ Carbon reactor from BRS and some of their Grannular Ferric Oxide and their Rox 0.8. Yea for me right?

I recently had a discussion with several people who have been doing this a lot longer then me. One of them stated the Rox 0.8 will leach PO4 and that he was able to prove it by simple demonstration. This of course made me question my choice in carbon.

Alright, so, here are the questions.

1. Do you run a dual stage and if so what type of GFO/ Carbon do you use?

2. What do you consider your normal and safe PO4 levels?

3. How far from those levels do you deviate before changing your GFO/ Carbon?

4. What type of PO4 test kit do you use? I'm currently using an Elos but am considering a change.

Thanks for the help.

Randy Holmes-Farley · Jan 6, 2015

That simple demonstration (assuming it is what people usually do) is generally not useful to show that phosphate from GAC is any sort of concern. In general, foods add so much more phosphate than does GAC that it isn't the primary place to look for a problem, especially not with a high quality carbon like ROX (which I also use).

I address it here:

Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both ? Advanced Aquarist | Aquarist Magazine and Blog

from it:
Comparison of Food Sources of Phosphate to Other Sources
What about other sources of phosphate, like the "crappy" RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.

Let's assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said, tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.

The same sort of calculation applies to analyzing other phosphate issues, such as the GAC in scenario three. The issue of finding "high" phosphate in GAC soaked in fresh water was frequently quoted as a reason to use one or the other brand of GAC, and probably still is. But simple analysis as shown above for the food rinsing puts the lie to this being a big problem.

One needs to consider how much GAC one will really use in the aquarium and how often it is added in order to interpret how important the added phosphate is. A typical recommendation might be 1 cup of GAC per 100 gallons of aquarium water, and to change it in 4-6 weeks. Let's assume we detect 0.5 ppm phosphate when a teaspoon is placed in a cup of water, and we get scared by the dark blue color during the test. Is this reasonable? That 0.5 ppm from a teaspoon in a cup of water translates to 0.015 ppm phosphate when a cup is used in 100 gallons.

That 0.015 ppm may be significant, being a typical target concentration level for reef aquaria and amounting to about half to a twentieth of the amount added daily in foods, but remember, it is used for 4-6 weeks. During those 4-6 weeks before the next replacement, foods add 50-700 times as much phosphate. So while it is not unreasonable to look for another brand of GAC, to blame phosphate or algae issues in the aquarium on its use would stretch credibility because it is a very tiny portion of the total phosphate being added.

Randy Holmes-Farley · Jan 6, 2015

For many years I mixed GAC (ROX) and GFO (BRS regular) in a canister filter. I now run them in separate BRS reactors to better control the flow (and my canister died).

I generally do not measure phosphate, but i go by the rate of algae growth on my glass as to when to change the GFO (about 1 week). The GAC stays for about 4 weeks.

IMO, ROX is among the best types of carbon for a reef.

TheDude · Jan 6, 2015

Randy Holmes-Farley said:
That simple demonstration (assuming it is what people usually do) is generally not useful to show that phosphate from GAC is any sort of concern. In general, foods add so much more phosphate than does GAC that it isn't the primary place to look for a problem, especially not with a high quality carbon like ROX (which I also use).

Wow, super fast response and tons of information. Glad I found this place. Thanks a ton for the info.

I figured this guy was going to the extreme with his crude test which was to test RO/DI water then drop a piece of carbon in the water and re-test it. No one else in the conversation seemed very concerned and in fact a couple kind of chuckled, they must have read the article you published. I'll get to reading it today, got to have something to do at work. lol...

Thanks again for the information.