Take a look at these. Assuming they have one close to the temp range you want it will be cheaper & in my opinion more reliable than something more complicated.
snap disc thermostats Make sure to get the right normally open or normally closed version for your situation.
I agree with David though. Hooking them in parallel will work except maybe for some weird super fancy thermostats.
Unless I'm completely missing D Nickolls point a diode won't do any good & probably would prevent your fan & pump from running. A diode can be thought of as a one way valve that allows current to flow in one direction only. Therefore, with a single diode in series with your loads, only the positive pulses or the negative pulses of the incoming AC would reach the load. At that point it is called pulsating DC. Reversing the diode would give only the opposite direction of pulses. Four diodes configured as a bridge rectifier allows both pulses to be used (100% duty cycle vs a 50% duty cycle with just one diode) but still only in one polarity. That is the first step in building a more elaborate DC power supply. There ... diodes 101 in a nutshell.
A good example of a diode at work in a permies situation. Consider a solar panel used to charge a battery. (for purposes of this example there is no charge controller) During sunny times this theoretical panel outputs 15 volts DC charging a 12 volt battery. Then at night the output drops until the battery voltage exceeds the solar panel voltage & the current tries to run backwards into the solar panel. This will destroy panels almost immediately. A diode placed in series prevents such damage because the current is prevented from reversing. Make sense?
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