Re: High levels of phosphates and nitrates

Douglas Duncan <duncand at sprynet_com> wrote May 21:

>I'm having a problem with high levels of phosphates in all my tanks, and my
>50gal chiclid tank which has no plants, also has elevated levels of nitrates.
>Phosphates combined with nitrates...guess what. Yep I've got an algae bloom.
>It's diatoms specifically. brownish red dusting all over the glass, heater,
>gravel, etc.. Low lighting may also be playing a part in it, but the phosphate
>is in excess of 10ppm and the nitrate is around 35ppm. Prior to this
>outbreak, >I recently did a delicate all day shifting of the entire
>contents of a 20gal >with fish, gravel, etc., to a new 50gal tank and
>hood. I'm thinking that >because I had to add a lot more fresh water than
>I would have during a regular >water change, plus adding more new gravel
>to the old, perhaps the bacteria >colonies were more severely smitten than
>I figured they'd be, and now I'm >getting some spikes on these elements a
>little like cycling all over again.
>Here's the question that qualifies this message in the plants mail list. What
>exactly are the optimum levels desired in planted tanks for phosphate,
>and iron? Recall that I'm having phosphate levels in my 75g and 50g planted
>tanks as well. The books I have and the test kit instructions don't come
>out and
>say for example, phosphate level should be 0ppm or 5ppm. Same thing with the
>iron. I'm pretty sure nitrates should be kept as close to 0 as possible.
>I don't think you want 0 phosphates as plants need a small amount to aid in
>photosynthesis don't they? If they get too much then they become damaged and
>algae grows better right? I have a lot of algae right now, and I didn't know I
>had phosphates til I got the test kit today.

Since nitrates and phosphates are the end products of bacterial oxidation,
I don't think that high levels of these two forms indicate failure of
bacterial activity, such as spikes of ammonia or nitrite do.  Bacteria can
oxidize ammonia or nitrite, but they don't do anything to nitrate unless
you have somewhere some anaerobic, reducing conditions, where bacteria will
reduce nitrate to atmospheric nitrogen, N2.  Usually, reducing conditions
release phosphate which has been tied up as insoluble iron or calcium
salts.  It is my guess that, if you did not have these high values of
nitrate and phosphate before you moved the contents from 20 gal to 50 gal,
the reason you are getting them now is because you had some plants in the
20 gal that were taking them up and are not doing that in the 50 gal.  The
best way to lower these nutrients is to get some fast growing plants, such
as Hygrophila polysperma or Ceratopteris, and give them plenty of light and
CO2.  In a few weeks, your tests kits will read 0, and you will be pulling
a lot of plant biomass out to take to the local tropical fish store for

Because very sensitive chemical tests for phosphate exist, a lot more is
known about utilization of that element in lakes.  It is known that aquatic
plants are extremely capable of removing phosphate from very dilute
solutions and can lower the concentration of phosphate to less than one
part per billion.  In fact, the uptake ability of plants is so good that a
given phosphate molecule doesn't last very long in a typical lake before it
is taken up.  The very low amount in the water turns out not to be a good
measure of how much is available, because there is rapid turnover.
Phosphorus is released by decay of organic matter and then taken up by
plants.  Turnover times in the order of 1 hour to 15 minutes in lakes have
been measured.  That means that in 1 hour to as short a time as 15 minutes
all the dissolved phosphorus  was taken up and replaced by an equivalent
amount released by decay.  Under these conditions, I doubt that phosphate
test kits available to hobbyists would measure any phosphorus, and yet, the
plants can be getting more than enough to support maximum rates of growth.

I have not been able to keep up with the literature on nutrient uptake by
aquatic plants, but I am convinced that aquatic plants are very good at
taking up nitrate, also.  (I am lumping algae and higher aquatic plants
together, here)  I have a Hatch kit that tests nitrate down to 1 PPM, and I
have tanks where I get a measurment of 0 PPM, then I add some nutrient
solution containing nitrate, getting imediately afterwards, a reading of
several PPM.  A week later, I again measure 0 PPM.  I have seen rapid
greening and growth responses in tanks where I have had visible nitrogen
deficiency when I have added enough nutrient solution to raise the nitrate
level from 0 to 0.6 PPM.

A number of people on this mailing list believe that they are controlling
algae by having a thick growth of 'higher' aquatic plants that get the
upper hand in the competition for phosphorus and probably other nutrients.
This may well be the case.  My only contention is that this competition is
occurring at nutrient levels way way below the levels that our test kits
can measure.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In hot, humid Mississippi where the Bermuda High has set up shop early and
kept away the rain.