When Is A Cycle Finished?

[Bignose]

Finally, let's say this lemon tetra is in a 20 U.S. gallon tank. That's 75.7 L. 0.000 012 434 kg is equal to 12.434 mg (multiply by 10^6). 12.434 mg/75.7 L = 0.164 mg/L and since at dilute concentrations a mg/L is roughly a ppm, this means that one lemon tetra in a 20 U.S. gal tank makes about 0.16 ppm of ammonia per day.

Well done on the maths Bignose...one thing though, mg/L expresses the same thing as ppm no matter what the concentration

No, you are mistaken here, a mg/L and ppm are the same only for dilute concentrations in water.

Let me give you a farcical example. Take 1 liter of pure ethanol. Since it is pure, that would have a concentration of 1,000,000 ppm. But, it I guarantee it won't be 1,000,000 mg = 1 kg. It will weigh about 0.789 kg.

a ppm is a like over like measurement. That is, properly a ppm should be a volume over a volume (like microliters in one liter) or a mass over mass (like mg per kg) or any other like measurements. Quality control of manufacturing process are often reported in ppm. E.g. a manufacturing error rate of 100 ppm, would be 100 units with errors per 1 million units manufactured.

mg/L is not a like over a like. it is mass over volume. It works out to be the same as a ppm in dilute concentrations of water because you can convert mass to volume using the density of water and a microliter of water at 25 degrees C weighs one milligram.

The density of ammonia is actually 694.2 kg/m^3 Compare that with water which is 1000 kg/m^3. So, there is a small error in there, depending on whether you are talking about ppm v/v (v/v stands for volume/volume) or ppm m/m (mass/mass).

If there were no interactions between the water and ammonia: 1 ppm v/v ammonia in water is actually equal to 0.7 ppm m/m ammonia in water. 1 ppm m/m ammonia in water is actually 1.4 ppm v/v ammonia in water. But, of course, nature doesn't work like that. There will be interactions betweens the ammonia an water to find the final density. The mixture density will not just be the linear combination of the two densities weighted by the amounts of each pure component.

So, this is where the phrase "in dilute concentrations" becomes important. Because, 1 ppm of ammonia in water is almost wholly water, the density of the mixture is almost exactly the same as that of water, so the conversion between mg/L and ppm works. But again, as an extreme example, 1 ppm of water in ammonia, or 999,999 ppm of ammonia in water will not be 999,999 mg/L, it will be about 694,000 mg/L.

To be completely exact, there are temperature variations in there as well. The density of water is only 1000 kg/m^3 at 25 degrees C and 1 bar pressure. At other temperatures, and pressures, the value is slightly different. There will be temperature variations in the interactions between mixture components as well. But, all these variations are small, well within the measurement error of our home test kits.

In conclusion, however, the phrase "in dilute concentrations" is critically important because in non dilute situations a mg/L is NOT a ppm.

 

[chipper_222]

I think things are being over complicated a little here. If you are dealing with water (as most on this forum presumable do!), they are equivalent. The reason is that 1 liter of water weighs 1 kilogram. So, when you have 1 milligram of a substance dissolved in water (weight of 1,000 grams or 1,000,000 milligrams) it is the same as 1 part per million. For other liquids this does not work out so neatly because they have different weights (which is what your getting at).

I agree with your ethanol and ammonia examples but when talking about fish tank water, which is an aqueous solution so therefore no matter what concentrations ppm is the same as mg/L.

Cheers

Chip

 

[andywg]

I think things are being over complicated a little here. If you are dealing with water (as most on this forum presumable do!), they are equivalent. The reason is that 1 liter of water weighs 1 kilogram. So, when you have 1 milligram of a substance dissolved in water (weight of 1,000 grams or 1,000,000 milligrams) it is the same as 1 part per million. For other liquids this does not work out so neatly because they have different weights (which is what your getting at).

I agree with your ethanol and ammonia examples but when talking about fish tank water, which is an aqueous solution so therefore no matter what concentrations ppm is the same as mg/L.

Cheers

Chip

But this is the scientific section where it is requested that people use particular language and post accurately. Stating that ppm and mg/l are the same is incorrect, as explained by Bignose. It might be a small matter, but the scientific section was set up for accurate (some might say anal) discussions.

 

[Miss Wiggle]

But this is the scientific section where it is requested that people use particular language and post accurately. Stating that ppm and mg/l are the same is incorrect, as explained by Bignose. It might be a small matter, but the scientific section was set up for accurate (some might say anal) discussions.

spoken like a true pedant

You are quite right though, we've had plenty of anecdotal discussions on this topic, the reason I raised it here is to get some hard figures behind the discussions so we can base what we advise on fact rather than anecdotes. For factual discussions we need to be accurate.

Also people like Bignose and Andy provide an invluable input to this forum in areas a lot of us flounder a bit, so we should let them have their idiosyncrasies

 

[iankent]

If my calculations are correct (worked out in excel using the molecular mass) I've found that:
1mg ammonia will become 2.701352mg nitrite, and then 3.640805mg nitrate, using 1.878905mg oxygen and 1.057821mg nitrate in the process (in addition to the nitrite/nitrate thats being produced, we're also producing hydrogen)

if we're assuming that 1mg/L ammonia is equivalent to 1ppm when added to water at 1 bar pressure and 25 degrees, and we're upping our ammonia to 5ppm from the start, just from the initial ammonia added we're going to produce 13.50676mg/L nitrite (assuming we have an unplanted tank, which other than ammonia and nitrite processing bacteria is sterile)

i'm going to stop talking in mg/L because its confusing me

So - day one and we have enough ammonia to produce 13ppm nitrite! As the ammonia is cleared, we add more to raise it to 5ppm again (to keep the bacteria fed) - all the nitrites are still there, because the nitrite bacteria has only just started to appear in decent numbers! We keep doing this until our tank is capable of clearing both 5ppm ammonia and the 13.5ppm nitrite in 12 hours. When we finally reach this stage, not only have we made the tank clear the 5ppm ammonia in 12 hours, but the 13.5ppm nitrites it's produced AND any remaining backlog from the cycle

if our aim is to clear the ammonia and the nitrites its produced, then we've reached this stage long before the nitrites in the tank have actually reached 0 (because of the backlog) - one way around this would be to do a water change every day, then top the ammonia back up to 5ppm immediately (no backlog, constant ammonia to keep the bacteria happy)

another way would be to wait until the tank is clearing 5ppm ammonia in 12 hours (thats the first part of our aim), at which point we should clear out the backlog of nitrites with a big water change then continue adding ammonia, which will keep the ammonia bacteria fed while giving the nitrite bacteria new nitrites to feed on! everyone's happy, but again there's no big backlog to clear first!

i've confused myself so much and changed the paragraphs around a bit, so apologies if I've not explained it as well as it sounds in my head

edit: havent looked this up, but do different nitrite bacteria grow when nitrite is found in high concentrations, as they do with ammonia? if so, then it could be actually slowing the cycle down, not just making it appear slower!

edit 2: just found this discussion, which seems to agree that different NOB's are produced with high levels of nitrite, similar to AOB's with high levels of ammonia - in which case, waiting that extra time for the high levels of nitrite to clear seems a little pointless?

 

[Bignose]

I think things are being over complicated a little here. If you are dealing with water (as most on this forum presumable do!), they are equivalent. The reason is that 1 liter of water weighs 1 kilogram. So, when you have 1 milligram of a substance dissolved in water (weight of 1,000 grams or 1,000,000 milligrams) it is the same as 1 part per million. For other liquids this does not work out so neatly because they have different weights (which is what your getting at).

I agree with your ethanol and ammonia examples but when talking about fish tank water, which is an aqueous solution so therefore no matter what concentrations ppm is the same as mg/L.

Cheers

Chip

chip,

what you just said there can be summed up into the phrase "dilute concentrations" which I used several times.. Like I said, a mg/L and a ppm, in dilute concentrations, are indeed the same. But the phrase "no matter what the concentration" is definitely wrong. Like Andy pointed out, this is particularly the subforum to be very exacting in the word choice. Not that I wouldn't have pointed out the mistake in any of the other forums -- because I know myself -- and I in all likelihood would have corrected it no matter what forum it was posted in. But, the fact remains that the way you phrased it is incorrect, and it important to stress the fact that a mg/L and a ppm are the same only for dilute concentrations.

 

[chipper_222]

what do you consider the term 'dilute' to mean?

 

[Bignose]

well, the definition of dilute will change from substance to substance, but normally it means less than 1%, 5%, or 10%. It really depends on the application. The concentrations of ammonia, nitrite and nitrate are always going to be "dilute" in that if they weren't dilute, the fish would be dead.

The conversion factor is only completely accurate in the limit of pure water, it just that the error in assuming that the water and what was diluted in the water having the same density is so very small that only the most accurate tests are going to detect it. The farther away from a pure liquid, the less accurate the conversion factor will be. This is obviously going to change from chemical to chemical.

Edited to add: Just to put some numbers on this actual discussion as well. A mixture of 1% by volume ethanol in water at 25 degrees C has a density of 989.9 kg/m^3. (This number can be gotten from the program AlcoDens, which calculates the mixture properties of blends of ethanol and water). This isn't a large difference from 1000 kg/m^3, but it is measurable. 5% isn't much, but it clearly shows that "which is an aqueous solution so therefore no matter what concentrations ppm is the same as mg/L." is just incorrect. The difference between 5000 ppm and 5001 ppm isn't 1 mg/L in this case, it would be 0.98 mg/L.

Now, is that a big enough error to even matter? Depends on what you're trying to do. If you are designing a reactor around the blend of ethanol and water, it might. If you are just testing your home brew for alcohol content, it probably doesn't. And, in terms of fishkeeping, all of our home test kits have huge variances in error, so a mg/L and ppm are going to be the same for the entire range of our test kits.

Nevertheless, I am technically correct (Futurama fans will know that that is the best kind of correct) when I say that a mg/L and ppm are not the same thing. There are only approximations that are very accurate for dilute concentrations. When you get too far away from a pure substance, then the approximation becomes more and more inaccurate.

 

[kj23502]

WoW! This is a great read!!! Thanks

 

[Miss Wiggle]

hmmmmm so in terms of practical advice, if someone is in this last stage of cycling where the last little bit of nitrite just won't shift then a way to tell if it's really ready for fish or not would be to do a significant water change (like 80/90%) to clear the backlog of nitrite, then add 5ppm of ammonia and test both ammonia and nitrite 12 hrs later, if both read 0 then the tank is cycled.

 

[iankent]

hmmmmm so in terms of practical advice, if someone is in this last stage of cycling where the last little bit of nitrite just won't shift then a way to tell if it's really ready for fish or not would be to do a significant water change (like 80/90%) to clear the backlog of nitrite, then add 5ppm of ammonia and test both ammonia and nitrite 12 hrs later, if both read 0 then the tank is cycled.

I'd agree with that Miss Wiggle - if the results are showing within the scale of the nitrite test then they should be low enough for the bacteria to clear relatively quickly (i.e., if your reading lower than 5ppm, then we're still within the targets of nitrite we want to clear), so this would only really be worthwhile if the nitrites are off the chart. Anything below 5ppm should be easy enough to clear with bacteria once you've reached the 5ppm-0ppm ammonia target - if just 5ppm nitrite isn't clearing in 12 hours then there's probably something else wrong

edited to add: just mentioned this on another thread, but thought it was worth mentioning here - if we do a water change to try and clear the backlog of nitrites, can you see any reason not to do a full 100% water change (to remove as much nitrite as possible?). I cant see there'd be any negative effect on the bacteria etc. in the difference between a 80/90% change and a full 100% change. Also, even once the water has been completely replaced, there'll be high levels of nbacs, abacs, ammonia and nitrite still present in the filter media which will be mixed back through with the water as soon as its changed! (Which is why doing a 90%+ water change still doesn't bring the nitrite readings back on the chart!)
(edited again to add, for clarification, I'm not referring to undergravel filters with the 100% water change idea!)

that leads to another question - if there are high levels of ammonia and nitrite still in the filter media (I'll test this out soon!), would it be worth waiting 10 minutes or so after a water change for it all to mix through, then doing more tests before adding further ammonia? if we add enough to raise it to 5ppm after a water change, that could actually increase it to 7 or 8ppm without us realising (especially if we add ammonia then test 12 hours later), which could contribute to slow cycling?

 

[Miss Wiggle]

no reason why you can't do a 100% water change, filter won't completely dry out in the time it takes to drain the tank and then fill it again. a lot depends on plants though, if you've got them it's best to leave an inch or two in the bottom of the tank otherwise when you re-fill, however careful you are, you're gonna mess up all your planting.

 

[waterdrop]

Hi folks, have been away from the forum over the US holiday weekend but have now had a chance to carefully read this thread (nice thread topic, MW, congrats!)

Have we reached any consensus here? My perusal leads me to feel that the participants have re-validated the 5ppm targeting concentration as a reasonable number having a little cushion and giving the beginner a good starting point for stocking, even if the stocking is close to beginner max.

I find it very interesting that this exercise seems to re-validate the current TFF fishless cycling article from a numbers standpoint. I recently tried to take a look at all the fishless method articles I could find out there at the end of web searches to see how they seemed to compare to our article. I'm sure many of you have performed this exercise over the years.

My feeling (best word I can give this exercise without beginning to break down specific areas of comparison with sources and data, which would require actual work!) was that our article is as good as I could find, in that the others often had things that were out of date or things that were left out.

~~waterdrop~~

 

[Miss Wiggle]

Have we reached any consensus here? My perusal leads me to feel that the participants have re-validated the 5ppm targeting concentration as a reasonable number having a little cushion and giving the beginner a good starting point for stocking, even if the stocking is close to beginner max.

I think so yeah, I'd like to see some more opinions/evidence regarding the second stage of cycling, getting rid of the nitrite. I'm thinking if you do a massive water change when your ammonia first drops to 0ppm in 12 hrs (cos at this point you'll have a fair build up of nitrite) then the NBacs can start fresh as it were and just process the nitrite from the daily 5ppm of ammonia rather than the whole backlog of nitrite. Then if it takes a week or so for the nitrite to start dropping maybe clear the backlog again.........

I want somewhere to set up another tank to tinker around with a fishless cycle!!!

 

[waterdrop]

Have we reached any consensus here? My perusal leads me to feel that the participants have re-validated the 5ppm targeting concentration as a reasonable number having a little cushion and giving the beginner a good starting point for stocking, even if the stocking is close to beginner max.

I think so yeah, I'd like to see some more opinions/evidence regarding the second stage of cycling, getting rid of the nitrite. I'm thinking if you do a massive water change when your ammonia first drops to 0ppm in 12 hrs (cos at this point you'll have a fair build up of nitrite) then the NBacs can start fresh as it were and just process the nitrite from the daily 5ppm of ammonia rather than the whole backlog of nitrite. Then if it takes a week or so for the nitrite to start dropping maybe clear the backlog again.........

I want somewhere to set up another tank to tinker around with a fishless cycle!!!

Well, if you'll recall, there've been quite a few recent fishless cyclers now where BTT & I have had them do both more water changes and also have had them cut the ammonia-add-amount way down right after the stage-change where ammonia first drops to 0ppm (not necessarily in 12 hours yet, iir), the experimenting being that we now assume the ABacs have a decent start and assume they will be ok on a "maintenance level" of ammonia while we try to produce less nitrite overall in the fishless tank so that we have a greater chance of getting some feedback from our nitrite tests somewhere along the long period we know we'll having with nitrite running high.

Basically this worked, in the sense that it allowed times during this long phase when the newcomer fishless cycler could see some feedback from the nitrite tests that some NBacs were there and were slowly beginning to be able to convert some nitrite into nitrate (you, of course can also test and see the nitrates.) Then, once nitrites are dropping to zero in about 24 hours, we had them ease the amount of ammonia back up from, say, 2ppm to 4-5ppm and go through the final phase at the usual 5ppm amount. In the long run, my feeling ends up being that nothing is really gained. The added complication of trying to explain this process to a newcomer more or less counter-balances the gain of seeing some feedback from the nitrite tests. And certainly, once one has been all the way through a fishless cycle for one's self and has "faith" that it really works, it is much simpler and easier simply to keep right on putting in 4-5ppm unless one feels like fiddling with it.

~~waterdrop~~