Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you keep the materials dry they keep for years. I keep them in sealed jars and mix up about a week's worth at a time, using clean, dry tools to measure them.
I use more K2SO4 than I do the others (KNO3, PH2PO4) but that is what works for me. (heavily stocked tanks, little or no K in the tap water) Looks like in your tanks the plants are already removing all the nitrogen that you add from fish food, so you will need to dose more.
As for how long you can stretch out the amounts, to make them last for years...
My suggestion is to start with a pound of each, and see how it goes. Each tank is different. Also, you might get more tanks :-) and start using the ferts faster.
VERY ROUGHLY there are about 2 cups in a pound of most of these, so if you are measuring by the teaspoon... 1 lb = 96 teaspoons. THIS IS VERY ROUGH!!!
So... if the EI method (or whichever one you are using) says to dose 1/4 teaspoon of something, then you get somewhere in the ballpark of 400 doses from a pound.
Water that comes from the source (well, river) with a fairly high GH is likely to have other minerals in it, including potassium, but this is not a guarantee. Are your plants now showing potassium deficiency? If so, then you will need to dose more potassium, and probably more than you will add with the other 2 fertilizers (KNO3 and PH2PO4).
If the GH in your water is that high because of something you added (Epsom salt, for example) that has no other minerals, then no, there is not potassium in the water. If the GH reads that high because you are using Seachem Equilibrium or Barr's GH booster, then there is K and a few other minerals in there.