Chances are it would run continuously trying to bring the temp down but failing, while running your electric bill through the roof. Also there is the chance that the continuous operation will cause the unit to ultimately quit all together.
The concept of a chiller is the same as an a/c unit for a house if you get an undersized seer unit it would never cool down you house it would run all the time as stated before. Also just like protein skimmers chillers are usually overrated in size they can handle so if max size is 30 gallons then prob it is rated for 20 gallons
Just get a correct rated chiller or over sized chiller as in the long run it will be cheaper as it will use less electricity
