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>Mine was doing the same thing, and responded to an increase in 
>potassium dose.  I put in several grams of potassium sulphate over a week
>or so, (160 L tank), and that is the only change I made.  I don't think
>that 0.25 mL of Flourish will provide any significant amount of potassium,
>and I think that if one uses KNO3 without K2SO4, either the nitrate
>climbs too much or one runs out of potassium. I think that one really
>should measure the potassium in the water - I don't yet.
>Paul Sears

I tested for awhile, but came to the conclusion that about 20-30ppm seems a
good range in a higher light fast growing tank and lower lighted crypt
tanks. I found that adding about 1/4 teaspoon per 20 gallons of K2SO4 in
addition to my regular dosing of fertilizers(TMG + KNO3 + fish food) on a
weekly basis did the trick for me. I have applied this "rule of green thumb"
with good results with R. macrandra.
I add the K2SO4 after the water change. Your results may vary but this seems
to be a good amount GENERALLY. I found it did well at lower lighting also (2
watts a gallon). The higher the light, the more K+,P etc you'll use so at 4
watts/gal perhaps adding another 1/4 teaspoon/per 20 gallons in the middle
of the week maybe a good idea if you have a high light tank. I added higher
dosages for the crypt tanks with lower lighting and didn't use so much
KNO3-if any.
The range of K+ can be a larger, more varied concentration than other
elements IMO. Therefore, testing often is not needed IMO as long as there's
a good amount in there. K2SO4 has done well for me.  Just a few weeks of
testing is about all most folks would likely need to do to get the feel for
dosing K2SO4 ......if that.
Worked for me. 
Tom Barr