When Is A Cycle Finished?

[Miss Wiggle]

OK, we've been having this discussion in the new to the hobby section the last couple of days, thought i'd move the discussion over to here to hopefully catch the attention of some of the sciency bods and try to get some factual contributions as discussions have been fairly anecdotal thus far.

When doing a fishless cycle, you usually add your 5ppm of ammonia, wait for the ammonia to drop down, keep then adding 5ppm of ammonia and the nitrite will spike and drop down. The usual criteria for the cycle finishing is that when you add 5ppm of ammonia and both ammonia and nitrite reach 0 in 12 hrs then the cycle is done.

There's often a very frustrating phase at the end where the nitrite drops down to about 0.5ppm or 0.25ppm in 12 hrs and just sticks there for a couple of weeks.

RDD say's (and a few people have supported this with anecdotal evidence) that when you reach this stage the tank is actually cycled and safe to add fish to. When you stock a tank you firstly will probably not get 5ppm of ammonia a day it may be more like 3/4ppm which the filter could probably handle and secondly because the ammonia production is not in one big dump it's a steady production through the course of a day the filter doesn't need to be able to process so much ammonia and the resulting nitrite at once.

discuss........

 

[andywg]

OK, we've been having this discussion in the new to the hobby section the last couple of days, thought i'd move the discussion over to here to hopefully catch the attention of some of the sciency bods and try to get some factual contributions as discussions have been fairly anecdotal thus far.

Well, the cycle is not a massively studied area, so it's not entirely easy to look at. I myself have never cycled a tank without fish, so cannot even speak from anecdotal experience.

When doing a fishless cycle, you usually add your 5ppm of ammonia, wait for the ammonia to drop down, keep then adding 5ppm of ammonia and the nitrite will spike and drop down. The usual criteria for the cycle finishing is that when you add 5ppm of ammonia and both ammonia and nitrite reach 0 in 12 hrs then the cycle is done.

Here is part of the problem. A cycle is truly completed at a time when the processing rate of ammonia and nitrite is equal to the production rate of the tank. Whilst it would be a pain, one would be better mixing up an ammonia solution and using an airline syphon with a check valve to have it constantly dripping in, though then we are in the realms of guestimating the amount of ammonia produced by fish.

There's often a very frustrating phase at the end where the nitrite drops down to about 0.5ppm or 0.25ppm in 12 hrs and just sticks there for a couple of weeks.

RDD say's (and a few people have supported this with anecdotal evidence) that when you reach this stage the tank is actually cycled and safe to add fish to.

Well, we know that nitrite is bad (and worse than ammonia, but only in that being shot is worse than being knifed), and that having it present is bad. Furthermore, as Bignose points out in a recent science thread, 1ppm ammonia is roughly equal to 2.2 ppm nitrite

When you stock a tank you firstly will probably not get 5ppm of ammonia a day it may be more like 3/4ppm which the filter could probably handle and secondly because the ammonia production is not in one big dump it's a steady production through the course of a day the filter doesn't need to be able to process so much ammonia and the resulting nitrite at once.

Indeed. Also, there are thoughts that in higher nitrite concentrations Nitrobacter spp start to colonise the filter rather than Nitrospira spp, so this too could have an affect, though I cannot be sure (as you have probably guessed, much of this is supposition).

The question being asked seems to be are we happy to put fish into a tank where 5ppm ammonia becomes 0.25 ppm nitrites in 12 hours. That is really a judgement call. Some might say yes, others no. Such a low level wouldn't bother me overly, but I would also have in mind the probability that my test kit is not reading entirely accurately.

So, a completely unscientific answer I'm afraid

 

[Miss Wiggle]

andy you let me down, i was hoping for all sorts of research and stuff from you

i agree that the cycle is not a true replication and ideally we'd be putting in a steady drip of ammonia over the course of a day to better simulate it. we were trying to convince someone to try a little experiment when they're at this final stage of fishless cycling, probably not by setting up air line but by putting in 0.5ppm of ammonia every hr or something along those lines to see if we do end up with any nitrite or if it is all processed.

i seem to "know" that 5ppm of ammonia per day simulates a sensibly but full stocked tank (like assume 1" per gallon rule is followed) but I don't actually know where this comes from. Has it been proven that this is the amount a fully stocked tank gives or is this just anecdotal?

 

[Tolak]

1 mg/L of ammonia becomes 2.7 mg/L of nitrite becomes 3.6 mg/L of nitrate. And since mg/L is pretty close to the same to as ppm, those ratios are the same for ppm.

That basically means that the daily consumption of 5ppm ammonia will lead to a daily production of 18ppm nitrate. In a week you will have 126ppm nitrate, a 50% water change will drop it to 63ppm. At the end of week two, you will have 189ppm nitrate, dosing at a rate of 5ppm ammonia daily. Same water change, 85ppm, in a week giving you 211ppm. How would you have to stock a tank to end up with results like these? Packed like a can of sardines comes to mind.

I always considered the 5ppm ammonia overkill, but it gives you a big cushion when you do start to stock a tank. A new bacterial colony is more fragile than a mature one, I always figured that if you are taking care of 2-3ppm ammonia daily you are good to go.

 

[Miss Wiggle]

1 mg/L of ammonia becomes 2.7 mg/L of nitrite becomes 3.6 mg/L of nitrate. And since mg/L is pretty close to the same to as ppm, those ratios are the same for ppm.

That basically means that the daily consumption of 5ppm ammonia will lead to a daily production of 18ppm nitrate. In a week you will have 126ppm nitrate, a 50% water change will drop it to 63ppm. At the end of week two, you will have 189ppm nitrate, dosing at a rate of 5ppm ammonia daily. Same water change, 85ppm, in a week giving you 211ppm. How would you have to stock a tank to end up with results like these? Packed like a can of sardines comes to mind.

I always considered the 5ppm ammonia overkill, but it gives you a big cushion when you do start to stock a tank. A new bacterial colony is more fragile than a mature one, I always figured that if you are taking care of 2-3ppm ammonia daily you are good to go.

hmmm never looked at the maths like that.

get asked every so often how you know when you need a water change, i've alwasy thought when nitrate has risen by 20ppm from the base level of your tap water then it's time for a change, on a sensibly stocked and filtered tank it'll be roughly weekly.

so 20ppm of nitrate converts back to 5.5ppm of nitrite, converting back to ammonia of 2ppm per week............

that's waaaaaaaaaaaaay off 5ppm a day. so if a tank can handle 2/3ppm of ammonia in a day and the resulting nitrite then surely that's plenty.

As you say though Tolak, the cushion is always nice and I'd rather beginners went into this all with as much 'breathing space' as possible. Just like we all know that you can stock a tank to more than 1" per gallon, but that's a nice easy to manage level for anyone coming into the hobby for the first time

 

[Tolak]

You also have to look back at some of the "New tank, help meeee!!!" topics, where a tank is fully stocked, with no consideration or idea concerning cycling. these tanks have 5-8ppm ammonia after a week, at a rate of 5ppm daily you would have 35ppm ammonia. You could clean your oven nicely with that.

 

[Miss Wiggle]

You also have to look back at some of the "New tank, help meeee!!!" topics, where a tank is fully stocked, with no consideration or idea concerning cycling. these tanks have 5-8ppm ammonia after a week, at a rate of 5ppm daily you would have 35ppm ammonia. You could clean your oven nicely with that.

yeah you're quite right Tolak.

I like this, challenging my assumptions.

 

[Tolak]

I'm waiting for someone like Bignose to come along and challenge my rudimentary high school math skills.

 

[Miss Wiggle]

I'm waiting for someone like Bignose to come along and challenge my rudimentary high school math skills.

lol, my maths is pretty appauling too, that's what excel is for though, can't do sums but i can write forumla's to do sum's for me

 

[backtotropical]

Very interesting thread, MW!

I mentioned recently in one of Stormy's threads that i thought that the 5ppm rule was set without the relevant knowledge and understanding of cycling, and Tolak has pretty much just proved that.

I do agree that the cushion is nice, but would be very interested to see someone cycle with say, 1 - 2ppm ammonia per day, then fully stock (or even overstock) the tank, and layout the results for us.

From the calculations above, 1 - 2ppm per day should be more than adequate.

I'd do it myself if i could convince the missus that another tank is a good idea. I'll keep trying!

Who'd ever have thought that TFF members would be leading the field in Fishless Cycling research, eh? Take a back seat, Hovanec!

BTT

edit: p.s. Tolak, your rudimentary maths skills look fine to me, other than where you say 85ppm and it should say 95ppm, and where you say 211ppm and it should say 221ppm.

 

[Miss Wiggle]

i agree, if i had anywhere to set up another tank i'd do an experiment. i'd only advise someone with another mature tank to do this so if you do see an ammonia/nitrite blip in the few days afterwards you can just bang some mature media in keep the fish safe.

 

[stormy78]

This really does make interesting reading. As MW and BTT know my tank has been cycling for my tank for 42 days so today is the end of week 6. The 4.00ppm ammonia is clearly processing easily within 12 hours however the NitrIte is taking a while to process and having tested it every hour last night it looks like it is processing within 17 hours. Im at the point now of trying to decide whether to mid stock the tank, eg 6 apistogramma's and 15-20 Rummy Nose Tetra's or carry on adding ammonia until both ammonia and nitrIte are processed within the 12 hours. This thread has swayed me more towards stocking the tank tomorrow but the last thing I want to do is harm any fish I add to the tank. As BTT and MW have pointed out its a judgement call..... I wish I had the time to add the ammonia every hour to raise it by 0.5ppm (thats about 0.9ml in my tank with homebase ammonia) to be sure it could handle the load...... I'm really not sure what to do.....

 

[Miss Wiggle]

This really does make interesting reading. As MW and BTT know my tank has been cycling for my tank for 42 days so today is the end of week 6. The 4.00ppm ammonia is clearly processing easily within 12 hours however the NitrIte is taking a while to process and having tested it every hour last night it looks like it is processing within 17 hours. Im at the point now of trying to decide whether to mid stock the tank, eg 6 apistogramma's and 15-20 Rummy Nose Tetra's or carry on adding ammonia until both ammonia and nitrIte are processed within the 12 hours. This thread has swayed me more towards stocking the tank tomorrow but the last thing I want to do is harm any fish I add to the tank. As BTT and MW have pointed out its a judgement call..... I wish I had the time to add the ammonia every hour to raise it by 0.5ppm (thats about 0.9ml in my tank with homebase ammonia) to be sure it could handle the load...... I'm really not sure what to do.....

for me personally i wouldn't take the risk and would complete the cycle to the 'usual' finishing point of 0 ammonia and nitrite in 12 hrs. However if i had a spare tank I would certainly try it knowing I could cycle the filter instantly with some matre media. But the evidence so far looks like it's swaying towards being safe to stock earlier than this.

v v tough call.

 

[Tolak]

Thanks for the correction BTT, that's what I get for winging some mental math after a long work week. Should have broke out the calculator, the one with an eraser on one end.

 

[Bignose]

Let's put some real numbers to this:

A fish's waste is proportional to its mass, so let's figure out its mass. Estimate the fish's length (L), width (W) and depth (D) and multiply them together, this give the volume of the fish (V_f) V_f=L*W*D. Now a fish's density is very close to that of water, so we'll just use that. The density (usually given the symbol the Greek letter rho=ρ) of water is 1000 kg/m^3. Multiply the volume of a fish by the density and you get the mass (M_f) of a fish. M_f = = ρ*V_f = ρ*L*W*D.

Now, like I said, a fish's waste production is proportional to it's mass. This is because most fish consume 1 to 2% of their mass in food every day. Most foods have around 40% protein content, and about 16% of protein is nitrogen, so, to estimate the mass of nitrogenous waste (M_N) a fish produces M_N =0.02*0.4*0.16*M_f = 0.00128*M_f This gives the mass of nitrogen molecules in the waste.

Since ammonia is NH3, not just N, we have to compare molecular weights. Ammonia's molecular weight is 17 and nitrogen is 14, so the mass of ammonia (M_NH3) is given by M_NH3 = (17/14)*M_N = 1.21*M_N

Lastly, since the ammonia is reported as a concentration, we divide the mass by the volume of the tank (V_t). Concentration of ammonia added by a fish per day = M_NH3/V_t

Let me do an example:

Let's do a lemon tetra. They get to be about 4 cm long, and I am just going to estimate 2 cm high and 1 cm wide. (I'll have more to say about this below.) I find it easier to work in m, since then the unit conversions are usually more straightforward, so L=0.04 m, H=0.02 m, and D=0.01m. That means V_f=0.000 008 m^3. Multiply that by the density (1000 kg/m^3) and you get 0.008 kg or 8 g for the mass of the fish (M_f).

Multiplying by 0.00128 to estimate the mass of the nitrogen in its waste gives: 0.000 010 24 kg of nitrogen in the waste (M_N) which is 0.000 012 434 kg of ammonia (M_NH3).

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.

Now, there is plenty of wiggle room in this calculation. Firstly, apart from a box-fish, most fish's volume's aren't going to be easily estimated by L*W*D. But, as has been mentioned in this thread, when dealing with estimating how much waste needs to be handled, I think that it is best to overestimate than under. Even if overestimating by severalfold, such as by assuming that a fish is a "box". I didn't mention it at the time, but I also went ahead and used the high value, for how much fish consume in a day. Some of the other variables include how messy of an eater a fish is, how quickly its metabolism runs, what the temperature of the water is, etc. But, my personal guess is that assuming the fish is a box subsumes a lot of those smaller errors/details by taking one take error in overestimation.

And, really, I think that the answers this method comes up with are pretty decent. A school of 10 medium sized tetras in a 20 gal makes about 1.6 ppm of ammonia per day -- that sounds just about right to me. So, 5 ppm may be overkill, but considering the stocking levels some people try to achieve, 5 ppm per day probably isn't too far off. Again, overestimation is highly preferred to underestimation.