co2 and water changes

Discussion in 'Carbon Dioxide (CO2)' started by davideyre, 15 Mar 2008.

  1. davideyre

    davideyre Member

    Messages:
    34
    Location:
    Oxford
    in a low tech tank very infrequent water changes are recommended on the basis that exchanging water causes fluctuations in co2.

    i know there is a tom barr piece of work that shows fluctuations in co2 resulting in increased algae growth. (i think this was in a high tech tank experiment??? or is there further evidence as well?)

    why then do weekly 50% water changes with EI dosing and injected co2 not cause a similar problem with co2 fluctuation every week? i imagine this is partly because with co2 injection and high light the rate of co2 turnover is much higher and so the effects of the fluctuation are shortlived.

    not wishing to question established wisdom, but trying to understand things a bit better.

    * if adopting a PMDD approach then large volume weekly water changes are less important as lower doses of nutrients are used. if water changes encourage algae via co2 fluctuations would it be better to do them less often if using PMDD?

    * what about new set ups where ammonia levels may be higher - often waters changes more than once a week are suggested, presumably this is because experience shows that raised ammonia with poorly established plants is more likely to promote algae than fluctuating co2?

    thanks.
     
  2. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    Hi,
    We need to be very careful when we compare the dynamics of a high light tank with that of a non-CO2 tank. The same principles are at work but plants in each operate at a different end of the physiological spectrum. The mechanism by which Carbon is "fixed" by plants for photosynthesis (in other words, the way in which carbon is stripped) is by use of an enzyme called Rubisco. As the ambient level of CO2 rises or falls the plants must either produce more Rubisco (or less of it) to react with the CO2. Unfortunately, Rubisco production is not instantaneous.

    Let's say you setup an injected tank and you set your needle valve to produce 30 ppm of CO2. If you then install new plants it will tank approximately 7-14 days before the the sensor, production and transport mechanisms in the plants can produce the correct level of Rubisco to match the CO2 level. Until that time the plant is photosynthesizing at an inefficient level. As long as you keep the daily level steady the plants will "come up to speed" and, when the level of Rubisco matches the CO2 level in this environment they can operate at peak efficiency. Water changes in a tank like this has no effect on the Rubisco levels within each plant as long as the CO2 daily stability is maintained. Since Rubisco levels are already high the plants can easily cope with this transient.

    In a low tech tank the plants are operating at the opposite end of the spectrum where CO2 is minimal, say 8 ppm. Under these conditions the Rubisco levels in the plants are low. Tap water typically is high in dissolved CO2, perhaps 15 ppm or so. You can test this by measuring the pH of a glass of cold water drawn from tap. If you let it sit for an hour or two it will de-gas. Take another pH measurement and you will see a significant rise. Since a water change is a transient event, and since Rubisco levels in the higher plants take so long to change there is no way they can respond effectively to this short term spike in CO2 levels. As they attempt to now produce higher Rubisco levels in response to the water change their efficiency falters. Algae however are easily capable of responding to this CO2 transient. Therefore, each time a water change is performed in a non injected tank the transient CO2 spike causes the plants to falter and gives algae a boost. Some algal forms like BBA sense CO2 fluctuations and can be triggered by this transient.

    In a high tech tank therefore water changes are necessary to remove organic matter and to lower the NH4 concentration levels, however in a low tech tank the organic matter is useful as fertilizer as it breaks down so removal is less critical, however water changes put the plants at a disadvantage. Barr suggests that a low tech tank need only have a water change twice a year since the plants will recycle the organic matter.

    Another myth that must be debunked is that water changes are somehow necessarily tied to nutrient buildup. This is not the function of the water change. As stated, organic matter removal is the purpose, therefore, the same water changes help whether you dose a leaner PMDD or PPS or EI. The battle against algae is closely linked to the removal of ammonia and the compounds associated with ammonia production within the tank. Nutrients have nothing to do whatsoever with algae therefore it is false economy to correlate the level and frequency of water changes to the level of dosing in a high light tank.

    In a newly setup tank it is always a benefit to do several water changes per week. The reason is that ammonia reduction in the tank is at it's least efficient state since the tanks population of nitrifying bacteria is at it's lowest. Since the combination of ammonia plus light is the reason we get so much algae during a tank startup, multiple water changes is a significant tool to combat this.

    Hope this clarifies.

    Cheers,
     
  3. davideyre

    davideyre Member

    Messages:
    34
    Location:
    Oxford
    thanks for taking the time to write such a long explanation. i follow well except the last paragraph but one.

    water changes will help remove ammonia and organic waste matter, agreed, and i can see why removing ammonia and sources of ammonia is important. therefore i can understand that the need to remove organic waste may set the frequency of water changes.

    however i am not sure that needing to limit nutrient build up is completely a myth (but by all means correct me if you think this is wrong). let me give an example: EI dosing is a estimate of requirements, we may over-estimate requirements. if excess nitrate is supplied at each dose beyond what is used before the next dose then nitrate levels climb. these levels rising may not cause algae, granted, but with time a point will come where the nitrate levels are toxic to fish. regular high volume water changes prevent this. therefore water changes may have a role in limiting nutrient build up, albeit for the sake of fish rather than plants. if this is the case the more that is dosed the faster nutrients levels may climb and the more frequently high volume water changes will be required.

    i can accept though that in a high light set up the build up of organic waste may be such that it is possible to make a case for high volume weekly water changes regardless of whether nutrients may build up to a level toxic to fish or not.

    thanks.
     
  4. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Messages:
    8,952
    Location:
    Chicago, USA
    Hi,
    The levels of NO3 which are considered toxic are extremely high and we would likely never see this level. Additionally, if one were to be concerned with buildup, one could simply stop dosing for a few days until the plant uptake reduced the nutrient levels.There would otherwise be no need for a water change. Many people forget this simple fact. Now what would happen if you stop or reduce dosing at specific intervals to neutralize the buildup and also stopped the water changes? You'd more likely than not develop an algae buildup. Both plants and animals appreciate water changes which functions to reduce the toxins associated with organic buildup. Our tanks are an unnatural closed systems with very small volumes attempting to mimic a natural open system which removes local organic buildup via inflow/outflow in the case of rivers/streams and large volumes in the case of lakes.Nitrate has gotten an undeserved bad reputation because it was the only thing measured at the end of the nitrogen cycle and so it was assumed to be the toxic byproduct of the closed system. As it turns out, fauna are much less concerned with the nitrate buildup than the ammonia it starts out as. :wideyed:

    Cheers,
     
  5. plantbrain

    plantbrain Expert

    Messages:
    1,950
    As with any planted tank, less light = less demand= less dosing = less work and this all can lead to less water changes.

    I think if you do something like an ADA style tank with low light, rich sediments etc, say 1.5 w/gal of light, some CO2 or Excel, once a month water changes are not an issue in the least. Add some algae eaters, fish for nutrients etc, dose conservatively(you do not have to test here even, remember, you have plenty of nutrients in the sediment still as a back up), targeting less rather than the actual demand, say add 10-20% EI 2x a week etc.

    This is easy and it's very resilient.
    It can handle more abuse than any high light tank might and require less work.
    And if things get away from you, they take a lot longer to cause an issue.

    I'd go so far as to say no dosing is needed if you have a rich sediment and low light with CO2 and good sized routinely feed fish load. It depends somewhat on the plants also, but you have most of the 400 or so species that float around the hobby available to you.

    A little more dosing and cleaning, pruning and light, and you can have most anything you want.

    But as far as it being possible, most things are, it's just what is your goal and what are the trade offs you are willing to give up for it?

    Some are pretty good like non CO2 methods and low light+CO2.
    High light+ CO2 is a good trade off for me because I like to grow them fast and also like to do test. I'd rather do a test in 3 weeks than 12 weeks. But I do not mind doing a water change 1-2x a week vs 1x a month.
    So...........
    Some would rather spend their time w/test kits and adding and balancing a dozen parameters.
    Some do not care either way, they just want some plants that grow in their tank, water sprite does a good job there.

    Regards,
    Tom Barr
     

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