Thanks for the clarification. I just wanted to be sure there wasn't some secret info about Catalina duds circulating that I was oblivious to.
Originally Posted by tuonor
In theory i) the excess heating capacity means it can heat the water to a higher temp relative to room temps (been way too long since I took physics but IIRC temperature differentials are proportional to energy applied, so if a 25W heater can increase a given mass of water's temp by 10 degrees, a 50W heater can increase it by 20) and ii) this will happen faster so you have less time to notice signs of distress in your fish.
Importantly, this is NOT intended to be a knock on the Catalina (mine has proven to be reliable, quite durable and generally the next best solution I've found), but its just a point on sizing heaters properly relative to tank volumes.
For a 12 gallon tank, 50W is probably just right if you're target temp is within 10 degrees of room temps.