Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Hard water means different things to different people.
To fish, hard water means high GH. Lots of Ca, Mg or usually both. Nothing to do with iron.
To someone doing the laundry hard water means poor suds and poor washing. Still has nothing to do with iron.
KH is Carbonate Hardness, and this is a buffer that stabilizes pH and is a source of carbon for some aquatic plants. Still, not related to iron.
CSM+B has about a dozen of the minerals that plants use in very small amounts. These are referred to as Micro nutrients "Micros" or as Trace nutrients, "Traces". This product may not contain all the iron that some aquatic plants need.
Other sources of iron may include the tap water (if the white bathroom fixtures are stained red-brown the tap water may have iron), fish food (but very little), substrate (especially an additive called laterite). If, among these possibilities the need for iron is met in your tank, then do not add more. However, most plants prefer more iron than is often available in these sources, so we add chelated iron. Chelation makes the iron more available to the plants for longer.
Yes, it is OK to add more K (potassium) to the tank until the plants show no potassium deficiency, and it is OK to add more than they need. Adding it as K2SO4 is safe in most conditions.
KNO3 is usually treated as if all it is adding in NO3. Test your tank for nitrate. Add enough KNO3 to read between 5-20 ppm. When I add more (NO3 tests over 20 ppm) I can see the fish are not doing so good. If you think of KNO3 as a source of NO3 only, the little bit of K that you are adding is not going to overdose the tank.
Similarly, if you need to add phosphate to the tank you will usually add so little KH2SO4 that the tiny amount of K does not count for anything.
Still, there is some K in these. Perhaps your tap water has more. If your plants are not showing potassium deficiency you might decide not to add more as K2SO4, just let the amount you are getting from the other fertilizers do the job.