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Potassium



I would like to get a second opinion about some advice I received over 
rec.aquaria.freshwater.plants.

Despite the fact that my tank is heavily planted with mostly fast growing 
plants, I am still unable to get my NO4 levels under control.  I have a fairly 
heavy fish load, but don't think I'm feeding them excessively for the population 
(feel free to verbally abuse me if I'm wrong).

The suggestion was to add potassium, either in the form of potassium chloride 
(salt substitute), or potassium sulfate.  I understand potassium is a 
macronutrient that I might be lacking,  but I have questions:

1 - Is lack of potassium a reasonable assumption, given my other stats?  How can 
I tell if I'm 'potassium challenged' (PC)? I've never seen a potassium test kit.

2 - If I am PC, should my plants be growing so fast? My Cabomba and Hygro. grow 
more than the height of my tank in ONE WEEK, every week, with the ambulia and 
pennywort not far behind!  I won't even describe the hornwort.  If I don't weed 
constantly, there's no room for the fish!

3 - How much do I use?  I don't want fish for dinner, even if they are well 
salted ;)

My stats are:

55 gal.
3W/gal flourescent
gravel only substrate (Monterey beach sand from LFS)
RO for make-up water, tap water (very hard) occasionally depending on tank's KH
established several months

6.8 pH
6 KH
60 ppm NO4 (!)
Kent freshwater plant micronutrient added regularly

fish: 4 discus (2 large, one juvenile, one baby), two 5" clown botia, 3 Ottos, 2 
zebra danios, 3 rams, two large but very well-behaved snails (one apple, one 
ramshorn).

food: 2-3 frozen gumdrops twice per day (4-6 total per day), plus a pinch of 
flakes

10 species plants: Hygro. polysperma, Hornwort, Ambulia, L. cardinalis, C. 
wendtii, Cabomba, Anacharis, Amazon Swordplant, dwarf Sag., Pennywort.

15+ species algae :\

 - Anthony