IMHO, refrigerant will only leak out of a system when there is a chance to escape to the environment thru means of refrigerant "eating thru" joints, valves, inappropriate refrigerant hoses used (not refrigerant friendly), etc. most points hv been covered by alvy oredi.
for aquarium chillers, normally there r 2 types. 1 using the thermoelectric technology, e.g. iceprobe, which is suitable for small aquariums. the other being the conventional refrigerant method which is the same as most household, commercial systems. in this case, using air cooled condensers rather than cooling towers (water cooled).