Grrrr, that does sound frustrating. I think I'd give a shot at a trace
element additive. I've tried many plant foods, but Kent Marine
Freshwater Plant Micronutrient Supplement is the only thing that ever
made much difference. I do know that I also have a fair level of
phosphates because I use phosphoric acid to drop my pH from my well
water's pH of 9.5. The plants tend to really use this up in a hurry in
my tanks, so it doesn't create an algae problem. Do you think you may
be short on a macro nutrient, like nitrates or phosphates? Most good
fertilizers don't add this assuming its all coming from fish food and
fish wastes.
Cary Hostrawser
My Rainbowfish Home Page
http://pw2.netcom.com/~caryho/home.html
Rainbowfish Study Group Web Page
http://home.stlnet.com/~gwlange/rainbowfish.index.html