What is a Peltier Thermoelectric Cooler?

Peltier cooler is often referred to as an electronic cooler because it uses a semiconductor chip that has hot and cold plates. 

When there is a current passes through, the chip will transfer heat between the plates for heat to be rejected or produced depending on the polarity of the plates. Peltier chip was discovered by a French Physicist Jean-Charles Peltier in 1834 when he passed a current through to product heat and cold energy from a junction composed of two different materials. 

What is a Freon Air Conditioner?

Freon air conditioner uses a compressor to compress gaseous freon into liquid state, then the freon is allowed to be cooled in a condenser for heat to be rejected to the atmosphere. After that, it is allowed to be expanded into an evaporator chamber either by means of an expansion valve or capillary tube. In the process of expanding from liquid to gaseous freon in the evaporator,  it absorbs heat from its surrounding. The cycle repeats when the gaseous freon is returned back to the compressor. 

What're the Differences?

As far as the quality of cooling is concerned, there is no difference between them. This is because water from aqua or reef tank will cycle through heat exchangers of the chillers. For freon chillers, titanium tubes are often used as cooling coil inside the evaporator whereas for peltier chillers,  polypropylene, aluminum alloy or stainless steel sections are used for heat transfer.