optimize peltier element cooling
A few comments and observations: -

A single TEC1-12706 peltier thermoelectric cooler can only cool effectively when the thing that it is attached to (the aluminium pipe) isn't conducting ambient heat from the surrounding environment back to the cooler at too high a level.

For instance the TEC1-12706 is rated about 50 watts and let us assume that all this power is converted to the cooling process. Let's say the aluminium pipe has a thermal resistance of 1 degC per watt i.e. if 1 watt of heat power were pushed into the pipe it would rise in temperature by 1 degC.

Or, put it another way, if 50 watts of heat power were extracted from the pipe it would cool by 50 degC. This would cool the pipe down to -25 degC in an ambient of +25 degC.

However, if the pipe has fluid flowing through it at ambient temperature then it won't cool down to -25 degC or even close because that fluid is imparting heat to the pipe and that means you need a bigger and more powerful peltier to extract the power due to the pipes basic thermal resistance AND the power that needs to be taken to cool the fluid.

Given that a Peltier cooler is probably around 15% efficient you have to consider that the heat power you can extract from an object is significantly less than 50 watts.