NO3 is actually the primary food source. Although it is a toxin in theory, it is almost impossible to reach a high enough NO3 level to kill fish.
I have very sensitive fish (Rays) and have had over 100ppm NO3 before. If you were to do 50% water changes weekly, I would bet you couldn't exceed 40ppm if you tried *within reason*. NO2 and NH3/4 are the toxins you should be worried about.
K can be anywhere from 10-30ppm and is so easy to hit with KNO3 that there is really no point in ever worrying about it. I dose KNO3 very heavily to the point where I have 28ppm per week K without any K2SO4 at all.
I would still keep the 25w bulb in there. I have 5.2wpg on my 50g and have no problems.
I PM'ed you about the dosing schedule creator. Let me know if you want it. I think it will help you out quite a bit.
Don't trust Iron testkits. They are frequently faulty. I like the Seachem one because it comes with a stock solution liquid that has a .1ppm/1gallon reading, so that you can test the testkit against a known solution before testing your tank. Dosing 1/2t of Plantex is about 1.75x as much as recommended for a 50g. That isn't bad, but is still excessive and should give you plenty of Iron. With my high light and co2 I still add Flourish Iron every other day. Seems to have helped lately.
edit>> forgot about the PO4. Dosing more PO4 may/will actually help with the algae problem. Once/if PO4 bottoms out you will see tons of algae starting to pop up. I keep mine at 2ppm with no problem. I noticed that my tank got a little funky on me when my PO4 hit 10ppm once, but I think I was begin stupid elsewhere at the time.
>> add your location to your profile that people see under your name. I bet you would be surprised by the number of people around you. Where are you from anyway?