Redfield Ratio

[CStrickland]

Hi chemists!

I've been noticing more posts about the Redfield ratio lately, and am looking for some advice on how we can use it in our tanks. Like, I get that when you grind up a bunch of plankton or whatev, you always come up with the magic number so that's cool. But what assumptions do you guys make from that / how predictive is it? Do you guys find it useful when your dosing or setting up tanks?

Thanks!

 

[disc1]

Like, I get that when you grind up a bunch of plankton or whatev, you always come up with the magic number so that's cool.

I wouldn't say it quite like that. Always is a strong word. More like, "you usually come up with something in the neighborhood of that number more often than not."

But what assumptions do you guys make from that / how predictive is it? Do you guys find it useful when your dosing or setting up tanks?

My favorite quote about it was something about it being the single most abused number in all of biology.

 

[dkeller_nc]

My favorite quote about it was something about it being the single most abused number in all of biology.

Yep, I wouldn't even consider "balancing" the concentration of carbon, nitrogen and phosphorus of my tank water by supplementing to achieve the Redfield Ratio, even if I had a ready means of analyzing TOC to get the carbon content.

My take on it is that it's instructive in the sense of showing that very little dissolved phosphorus is necessary to fuel the growth of various critters in a reef tank.

 

[CHSUB]

My tank always seems to look better and corals seem to grow faster when my n and p are balanced around Redfield. Maybe it’s just coincidence and as long as nutrients are available in some low level corals will do fine? I never add n or p to achieve some magic ratio, however there was a time I dosed po4 because level was unreadable. Now with 22 fish and five a day feedings level stays around .01. I have noticed many experienced reefers, including myself (CHE), recommend po4 at .03 or lower and no3 at .5 or lower. There are studies that confirm n/p ratios of 10/1 and less promote cyano. This could also explain why higher levels of no3 and po4 work for some and don’t work for others. If you look at the thread in the SPS Forum, “guess the po4 level” his ratio is right around Redfield.

 

[RussIV]

if you are truly ulns, you will see that supplementing po4 and no3 as needed will make a much happier tank.

if i let my tank go for 2 weeks i will have undetectable no3/po4. corals will pale and then die.
if po4>no3 i see gha and cyano as do most people who achieve the same with carbon dosing.

but by all means, im the guy you hate lol

 

[CStrickland]

Yeah, that's what I figured. Like it's more for explaining the results you see than another number to chase, especially since we aren't measuring carbon. I guess I don't really see why it would matter what the balance is until your numbers are vanishingly small. Assuming sufficient C, if there's any nitrate, your biofilter is going to use it to pull 1/16 that much phos. So if the ratio was all there was to it you'd never see tanks with 2ppm phos and 50ppm nitrates cause they would have automatically shaved down to the last phos available. In other words you'd only ever have high nitrates OR high phos. What does make sense in that scenario is carbon as a limiting nutrient, and that's exactly what we see when people start dosing C: nitrates and phos plummet according to C dose.

Also, wouldnt dosing phos be less effective than just feeding more because the coral eat it in the organic form? Like if you have zero but you feed enough of something pretty unprocessed, then the smaller pieces and fish poop is gonna be a Redfield-balanced meal for the coral. Sometimes bb tanks get underfed, or people go crazy with the gfo, but thats an easy enough fix without the algebra.
I guess it might be smart to pick foods that are Redfield balanced, so you are adding the right ratio to the tank. But again, isn't that not going to translate into the water test results if the fish are storing phos as atp (?), and pooping out nitrate heavy waste? At least until the fish dies and rots...

@csub, how do you balance phos to nitrate when the cyano can fix its own N? Isn't it still just a phos-reduction project?

 

[Dan_P]

Hi chemists!

I've been noticing more posts about the Redfield ratio lately, and am looking for some advice on how we can use it in our tanks. Like, I get that when you grind up a bunch of plankton or whatev, you always come up with the magic number so that's cool. But what assumptions do you guys make from that / how predictive is it? Do you guys find it useful when your dosing or setting up tanks?

Thanks!

Ecological Stoichiometry is what came after the pioneering work of Redfield and using C:H:N ratios to explain predator prey interactions and growth efficiencies looks useful. The latter is a useful topic to be familiar with because it covers how much biomass is generated per the amount of food consumed. Optimizing the food C:H:N ratio to that of the consumer's C:H:N ratio optimizes growth efficiency. The Redfield number could be a useful if you knew what nutrients were currently available to an organism and you wanted to changes them. Growth efficiency also explains why our systems don't become clogged with biomass. A lot of food is converted to CO2 and lost, though we do see PO4 and somtimes NO3 accumulate.

Adjusting nutrient stoichiometry is the basis of carbon dosing to induce bacteria growth and fertilizing photosynthetic organisms. Fish grow better with the "right" food is in part related to matching food and consumer stoichiometry. Dosing for coral growth makes sense if you knew what they had available before dosing and then added the right stuff to balance the stoiciometry, otherwise, it's guess work and falling for marketing claims.

Using stoichiometry to adjust dosing could be useful, but lacking the C:H:N information for the animal of interest forces you to use something like the Redfield number as a starting point and observing the growth as a guide for further adjustments.

 

[billsreef]

My favorite quote about it was something about it being the single most abused number in all of biology.

Indeed. The Redfield ratio comes from open oceanic waters. It has since been discovered that different organisms have widely different Redfield ratios. The only thing you can really take away from this for use in our tanks is that more carbon is used then N and more N is used then P. The numbers are far too variable to be of use to determine just how much of anything to dose in order to effect anything else. Best you can do is experiment a bit and figure out just what works in your tank, which will be different in my tank, which will be different in disc1's tank, etc., etc.

 

[disc1]

My problem with using it to dose by is that the ratio comes from the makeup of the bodies of the organisms, not from the water that best supports them. It is foolish to think that matching external conditions to internal conditions leads to optimal conditions.

 

[rovster]

In my tank when nitrate bottoms out and phosphate climbs the cyano shows up. I dose nitrate and phosphate drops with no extra gfo and cyano disappears. Gone through it too many times for it to be coincidence. I don't try and balance anything to some specific value or proportion, but rather keep my nitrates readable via nitrate dosing as needed. When I do this, tank looks noticeably better and corals look happier across the board.

I'm still trying to figure out how to stabilize the nitrate levels. Seems like a constant balance between too much and too little. Wish I could dial it in like I do with Alk. One day, lol!

 

[CStrickland]

@rovster - do you dose carbon?

 

[rovster]

Yes. Combo vinegar vodka. Lowish dose too.