Haha, "read the guide" "read the guide"... you guys... *sigh* lol
Rex, I know you get similar questions several times a day and it takes awhile to repeat yourself...
Oh well, let's break it down and see if I can help:
Why doesn't the wpg thing not work with smaller tanks? What does this mean for me?
WPG does not cover all the bases - it works best in the 10 gal - 100 gal range and doesn't apply as much outside those ranges. It also doesn't take into account things like how well your reflector works, how tall your tank is (makes a big difference) and the LUX rating on your lights. Power compacts tend to work better for longer than standard output florescents, so they don't really compare well. What does this mean for you? Well, stick to plants that can handle having only 18 watts 8" above them (I'm guessing on the height of your tank). I'll give ya a good list.
My riccia is growing well, and it needs higher light than the vals.....so why wouldn't my vals be doing fine? Am I supposed to stick to the low light plants then?
There are several reasons the vals arent' working well. They get tall (I've had a V. americana with 8 foot leaves before), so too big for your tank. They like harder water (I grow mine in tap water - 8.4 before I add CO2, liquid ROCK!), so your pH down isn't helping them. Sometimes if you try a plant and it doesn't work, don't sweat it - just try something else and move on.
I did find more about dosing chemicals (nitrogen, potassium, phosphorus, and iron by Flourish) Do I need to buy all 4?
Plants DO need these things to grow and be healthy. You may already have sufficient potassium in your water if you use potassium salt in a water softener - in which case, dont' dose it. The others you're going to want to dose and keep within the recommended ranges (20 ppm for NO3, 1-2 ppm for P04, and dose as per the directions for the iron, since test kits for that are pretty much useless).
Also, if I'm not buying a test kit for potassium and iron, how do I know whether I need to dose or not?
Potassium and iron test kits are worthless. This is where the EI method comes in - the lovely thing about it is that it eliminates the need for testing. You add enough of these things so that you KNOW there is enough, and then you do a water change ever week so that you KNOW there's not too much, and it isn't building up!
So, are you saying that I only need to buy the phosphorous test kit (i already have nitrates).....and also buy the additives for all 4 - nitrogen, potassium, phosphorous, and iron?
Depending on your water softener, yes. Check out http://www.gregwatson.com/
- it will tell you exactly how much to buy to dose what, and Rex's Guide (haha, again you hear it - sorry!) tells you how much to add.
Hope this helps. Please feel FREE to ask more questions.