when I save and quit and reopen the world my two dynamic tanks would be empty.

Please don't just delete the format, especially when you aren't going to even bother providing all the information it asks for. What version of mekanism is this with and what version of minecraft

You must to remove a Block from the Tank and place it then the Fluid is back
But you must to do this again and again when you restart the world

Minecraft 1.12.2
Forge 14.23.5.2847
Mekanism 1.12.2-9.8.3.390

sry for Error i have use a translator

Is this occurring in the 1.15 dev builds?

i dont know
I do not have 1.15 installed
But what I forgot to say is that for me it does not concern both tanks but only one tank

My deuterium tank in my base is not affected by this problem only my tritium tank in the desert is affected