[gigiggufh]

Fluid is a Latin root, so why not "fluidostat" on the model of "aerostat"?

[4gotunameagain]

because -stat is a Greek root.

merriam-webster

   borrowed from French aérostat, probably back-formation from aérostatique "of aerostatics," with -stat (after héliostat heliostat) taken as the Greek agentive element -statēs "one who causes to stand" — more at -stat