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Low PO4 Levels & Miracle-Gro house plant food

Tom wrote:

> My tank is finally starting to look good, thanks to the help many of you
> have provided to me.  However, I would greatly  appreciate any
> help that you experts can provide to me interpreting some test results,
> specifically the PO4 levels in my tank.  Results are as follows:
> PO4 = 0.20
> pH = 7.8
> GH = 7.5
> KH = 4
> Fe = undetectable
> Ammonia & Nitrite = 0

I wouldn't call 0.2ppm PO4 a low level of phosphate.  Why do you maintain
your pH so high - is it by design?  At that combination of pH and KH your
CO2 level is only 1.87gm/l.  Do you inject CO2?

> In the past, I have used the liquid fertilizer very lightly
> because it seemed to cause algae outbursts.  My plants are growing
> more quickly now, and I have very little (almost no) algae at all -
> except for the little spots that develop on the glass.

In my experience PO4 is dangerous stuff.  Using it in conjunction with other
nutrients is important because any phosphate that is not consumed by your
plants, perhaps because they are unable to due to a deficiency with another
nutrient, can be scarfed right up by the omnipresent algae.  Do you know
what your nitrate level is?

> After each water change, the plants grow very quickly for a day or so ...
> then slow until the next week.

My guess is that just after a water change you hit the sweet spot - that all
your nutrients are balanced correctly for your plant load.  After a day or
two the plants have sucked up the nutrients and something is limiting their
growth.  Do you test for nitrate and phosphate at that point?  If so you
might be able to figure out what's missing.  Does your KH dip at about the
same time?  I'm mystified by the low CO2 level.  I wonder where your plants
are finding the CO3 they need for growth.

> Should I be adding a fertilizer with PO4?  How about my liquid Miracle-Gro
> house plant food with Cleated Iron? (guaranteed analysis 8-7-6)

Hard question to answer.  It seems like you've found a balance-point.  Any
change to the components that make up that balance can throw it out of
whack.  Change things slowly and carefully, and measure with good quality
test kits at every step along the way!

michael rubin
michael at rubinworld_com