You're not looking for a computer. You're looking for a thermometer and a PH tester. Unless you want to control ozone, dosing pumps, Co2, lights, heat, pumps, wavemakers, pager/txt alerts, logging...
If you really want one, here's a couple year old comparison chart of them: http://www.aquariumcontrollers.com/aquarium-equipment-comparisons.php#controllerchart