My previous experience with the Jobe's Fern sticks was cutting each stick into approximately 8-10 very small pieces with a razor (very carefully, they snap!), then I would just plunge them into about 2.5"-3.5" of onyx down into a very thin peatmoss layer at the bottom. The initial growth was actually very good, you could certainly tell which plants had direct access to the tabs... I can only presume that during vaccuming and substrate disruption that some broken down bits leeched out. I never could manage to find the old ones to pick out... probably due to the small size.
"Would the use of spikes or Flourish tabs help prevent vals from leaching KH out of the water column?"
Well, that's a good question... Firstly, I think we need to answer if Vals prefer to get their calcium via the roots or water column if both are offered... Does anyone know this? I've always used a little bit of oolitic aragonite sand mixed in with any substrate I plan to plant Vallisnaria in. From experience, it seems to make the difference between fair growth and explosive growth.
Secondly, I am uncertain of the makeup of Jobe's Fern sticks, however here is the makeup of Flourish Tabs right off the box for you (They do appear high in calcium.):
Total Nitrogen (N) 0.28% (Water Soluble)
Available Phosphate (P2O5) 0.17%
Soluble Potash (K2O) 0.16%
Calcium (Ca) 14.9%
Magnesium (Mg) 0.06% (Water Soluble)
Sulfur (S) 12.2% (Combined)
Boron (B) 0.029%
Chlorine (Cl) 0.55%
Cobalt (Co) 0.001%
Copper (Cu) 0.001% (Soluble)
Iron (Fe) 2.2% (Soluble)
Maganese (Mn) 0.23% (Soluble)
Molybdenum (Mo) 0.0009%
Sodium (Na) 0.14%
Zinc (Zn) 0.0024% (Soluble)
(Derived from: Potassium Chloride, Calcium Sulfate, Copper Sulfate, Maganesium Chloride, Ferrous Gluconate, Cobalt Sulfate, Magnesium Sulfate, Manganese Sulfate, Boric Acid, Sodium Molybdate, Zinc Sulfate, Protein Hydrolysates)
Hope this helps!