i measured the properties of my tubes and got my capacitance calculated. for all 4 tubes it should be 0,63nF, with my l/c-meter i measured 1,00nF, so this could be right more or less. unfortunately i do not know how to calculate the parasitic capacity of my coil which would make the whole capacity larger.

another problem i do have with my coil: i measured 1,23µH for each one with my l/c-meter. but i calculated a capacitance of now 25µH for both in series. however i do not know the relative permitivitty of my ironpowder core, a table here says it's between 300 and 3000. taking the minimum of 300 leaves me with 7,6mH. Ohmic resistance of the coil and the wires to the cap is 2,6Ohms.

As a dielectric constant for water i take 77, the lowest value too. All my caps together should have ~50nF while filled with water.

L,C,R bring me to a resonant frequency of 3,2kHz, but again i do not know. The µr factor alone could change the frequency tremendously.