+1 Gotta figure out another way to keep your tank cool.
Say you put in a 2L bottle of frozen water. I reckon that would lower the temp in your 50L tank by around 1degree and you would have to do that repeatedly throughout the day to keep it down... And the fan you have there says on the box can only do max 4 degrees (depending on circumstances, humidity, temperature, etc.)
So, it won't be enough for middle of summer in auckland. Gotta get a chiller.