Long post ahead. Plan accordingly.
I'm not sure that it has really. A lot of people that do CO2-injected planted tanks will do a 'silent cycle', which is essentially a fish-in cycle with plants and a light stocking with fast growing plants, wherein the plants consume most of the ammonia produced. There remains a small amount of ammonia, below the detection limits of liquid test kits, that will cycle the filter. Remember that in the initial stages of cycling, the ammonia consumption is exceptionally small. Eventually the bacteria will overtake the plants for ammonia consumption and the cycle repeats for nitrite. I'm not sure that I've ever heard of CO2 per se stalling a cycle. Considering the fact that some people inject CO2
directly into their filter, I don't think it is an issue.
Napkin math gives about 17 ppm KH per 2.5 ppm ammonia or 1.5 dKH (27 ppm KH)/4 ppm.It's pretty obvious where people can get into problems with buffer consumption with this level of repeated ammonia dosing. That being said, people with planted tanks should be doing water changes, which will be replenishing buffer. Same thing with fish-in cycles. This certainly raises a problem with the 'no WC' philosophy of fishless cycling.
I think most of the help threads in our 'getting started' section fall into one of a few categories: nutrient limitation, pH 'crash'/buffer consumption, nitrite toxicity, 'user error', psychological.
Nutrient limitation is exactly what you think it is with regards to phosphate block. The only thing I'm saying in addition to this is that the number of potentially 'limiting' nutrients extends well beyond phosphates to include potassium and various metals/micronutrients, eg iron. That being said, the majority are probably phosphate due to the large phosphate demand of a rapidly proliferating bacterial colony. Phosphate is used for many things in a functioning cells, notably in DNA synthesis which a proliferating bacteria colony will be doing a lot of. No phosphate = no DNA for new bacteria. Iron, sulfur, magnesium, potassium are all also needed in less significant but non-trivial amounts. Unfortunately, people with soft water (as well as others) might have very low levels of these minerals available, as might a planted tank without fertilization wherein plants will take up much of the available nutrients (WCs will reduce or eliminate this). This kind of 'what am I missing?' problem comes up all the time on the planted side of the hobby. I have seen multiple cases of people cycling tanks fishlessly with plants that present with very classic phosphate deficiencies in plants not particularly susspetible to nutrient issues such as anubias (eg
here).
I think that a 'pH crash', so to speak, is a real possibility in tanks with low to moderate alkalinity that repeatedly redose ammonia to a higher concentrations. Fortunately, it is pretty easy to figure out if this is the cause, as pH tests are pretty common these days. It would be interesting for someone to monitor KH over a cycling period though to see if the drop in alkalinity is roughly in line with what we predict mathematically.
Nitrite toxicity... It's just what it sounds. If you've dosing 4 ppm of ammonia daily, you'll create 75 ppm of nitrite in a week's time. The longer the cycle takes, the higher levels take. I'm willing to argue that 12-20 ppm of nitrite won't cause toxicity issues, but 100+? Much more likely.
User error is... probably much more common than we realize. I'm putting things like improper water dechlorination, ammonia with surfactant, improper use of test kits, etc, in this category; things that we wouldn't necessarily think to ask and that posters wouldn't mention that they are (or aren't) doing. If you consider that many people are that are fishless cycling are completely new to the hobby, it's completely reasonable to expect them to make so called 'beginner' mistakes. This might be a cynical perspective on the matter, but it happens more often than you would think.
For the last one, I think a little background might be in order to fully understand where I'm coming from.
Everyone on this forum brings their own background to the hobby, even if it's just a perspective and not necessarily direct knowledge of the hobby. In my case, I bring a science background (chemistry, biochemistry, and biology mostly) as well as a medical perspective. I'm constantly seeing parallels between the aquarium hobby/participating and clinical practice. Let me give you a few examples.
In case #1, I have someone come in for a yearly checkup. Now, if I were so inclined, I could run a battery of unindicated tests, I will eventually find
something outside of normal ranges. I can then run a bunch more tests to work up this incidental finding to make sure it's not something important, but it's A) a waste of money and resources and B) incredibly taxing on the patient from a psychological perspective. To this extent, we caution against running unnecessary tests that don't have much benefit or direct indication. How does this relate to aquaria? Liquid test kits are the MRI of fish tanks. You see a lot of posts on this site that are essentially "Help, my pH is 8.2, how do I get it down?" This hypothetical person probably tested his tank for really no reason other than he could, found something outside the 'normal range', and is now worried about the health of his fish. In reality, his fish are probably well adapted to the somewhat higher pH and are doing just fine. The flipside of this is if he's having trouble with Rams (or something) dieing in his tank, and he tests his pH to find that it's 8+. At this point, there's a reasonable reason to intervene. Same thing with fishless cycling. People will test their ammonia
every day for the first two weeks to see if their tank is cycling. It's just obsessive really. Similarly, you give someone enough test kits and they
will find something out of whack with their tank. High GH, high phosphate, etc, and they'll worry about if this parameter is harming their fish. A more medically-minded method for testing in a fishless would be to wait until the three week mark and check ONLY nitrite. Nitrite showing up? Fine, put it away the test kit for a few more weeks. No nitrite? Check ammonia, check nitrate. That will let you know if you've cycled early or have a hangup somewhere.
TL;DR: Put down the test kits, step away from the tank, breath.
In the second example, we have a process called 'watchful waiting'. In essence, this means that we've observed something that's could require intervention, but may resolve on its own or not need intervention for some time. Examples of this would be some slow growing cancers, like some forms of prostate or DCIS. Another example would be a sore throat that looks viral (no antibiotics needed). People are often impatient though, and want surgery/antibiotics/whathaveyou. Same thing with cycling a tank. Say your test comes up with no nitrites at 2 weeks (which is pretty common judging from submissions to this forum). At this point, I think the best course would probably be to check for red flags (ammonia with surfactants, cycling in RODI, etc) but otherwise reassure the poster until 3-4 weeks without ammonia. A lot of tanks are 'late bloomers', if you will, and take 2+ weeks to cycle while others see ammonia drops in the first week. Unfair maybe, but no reason for worry until later.
I guess what I'm trying to say in the rambling dialog is that many,
many cases of 'stalled' cycles are actually within the realm of individual variation and very possibly not stalled at all, but rather a result of the poster's anxiety. I took a quick survey of the last 11 fishless cycle 'stall' threads and attempted to classify them as to the nature of the problem, as I saw it. I got one case of 'user error', one that was bonafide 'stalled' (4 weeks w/o ammonia), one that could very possibly have been nitrite toxicity, and 8 that were either cycling normally or within reasonable tank to tank variation.
Most of the mineral content in ground water is from the water stripping it from surrounding rock and minerals. The ideal is that water will pick up other minerals from the ground in proportion to how much calcium/magnesium it picks up. Seems reasonable enough to me, although it might not be the case. This would really be a question for someone with a stronger geology or limnology background that me.
I would mostly agree. I might add 2 ppm or so every week or every other week for peace of mind though, especially if you don't have anything else in the tank.
