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You need an osmotically neutral solution to keep them submerged for long, I'll try to find the recipe. Yes they sense pH and salt concentrations
From Dirasat, Pure Sciences, Volume 34, No. 2, 2007
Effect of Lead on Growth,Viability and Structure of the Slime Mold Physarum polycephalum
Ziad A. Shraideh *
Physiological Salt Solution (PSS) 6.0 mM NaCl, 3.0 mM KCl, 1.0 mM CaCl2 ,0.1 mM NaHCO3 ,0.5 mM MgCl2.
You could try getting a bottle of sterile eyewash and trying a few dilutions of that - roughly 10% might be a good starting point
Not sure I follow, Palladium?