I love it when an internet expert twigs onto an idea and presents it as though he has personally and single-handedly discovered a Unified Field Theory, a guaranteed path to world peace and a cure for the common cold all rolled into one. I can't believe that
RD.
didn't watch this fountain of Hidden Truth. An opportunity for enlightenment like this doesn't come along every day.
Well, okay, I guess such a moment comes along at least a few times every day on the internet, but...oh, never mind...
Now, at risk of upsetting this revolutionary applecart, let's take a look at this. I'm no mathematician; if I were I could probably boil this down to a simple equation, but I'm just gonna look at this from the Neanderthal perspective.
Let's say, for the sake of simplicity, that you do a 50% water change every week. Let's also assume that your bioload creates a completely stable and consistent amount of ammonia each week which equates to 10ppm of nitrate in your cycled tank. Just take a pen and paper and quickly scratch down some numbers; you will see that in this case the amount of nitrate remaining after your 50% water change will increase by a smaller and smaller amount each week. After your 10th weekly water change, you will have reached an equilibrium, at which point your tank will have 20ppm before the next change.
You will then remove half...bringing you down to 10ppm...and then your fish over the next week will produce another 10ppm, bringing you back up to 20 at the end of that week. You then take out another 50%...i.e. 10ppm...bringing you back down once again to a post-change level of 10ppm. And there you will stay
ad infinitum, with your water creeping up to a level that is twice what your fish produce within a week, after which you will knock that down by half only to have the dang fish eating and pooping and respiring to bring you back up to the same level you were at the previous week...which is the same level you will be at
every single week going forward.
The actual numbers here will vary depending upon how much your fish actually produce per week, and also upon what percentage you change and how often, but the same thing will hold true. You will reach an equilibrium point...not in years or months, but often within just a few weeks...and the biggest question will be what your feelings are regarding nitrate toxicity. Our hero in the video seems to think that 40ppm might be the time to start getting nervous, while others might choose higher or lower levels. I'm not even going to open that particular can of worms.
Another variable factor is the bioload in your tank...which is almost never completely stable and consistent. Fish grow...they die...they are replaced...they breed...feeding levels change slightly...plants grow and die and are pruned, changing the amount of nutrients which they utilize...water changes may be slightly inconsistent in their timing and/or their volume.
Nitrate creep is not something that continues upwards forever. It reaches a plateau level, at which it ceases to rise...if nothing else changes. But most changes conspire to increase the bioload, which then moves that plateau level higher and higher, eventually into dangerous levels. The greater the volume of your water changes, the longer it takes to plateau, but it will still do so. Our buddy in the video got something right; an occasional extra water change will help slow this accumulation.
Remember: common wisdom notwithstanding, there is
no such thing as "old tank syndrome". The actual problem is
old water syndrome, and the solution is simple; get rid of the
old water and replace it with
new water...a lot.
