Review on Mini Fluval Marina Submersible Aquarium Heater: Effective Heating Solution for Fish Tanks by Nick Worley

Choose a 5 gallon Marineland Portrait with good circulation. I chose the 50w size because it was recommended for aquariums up to 10 gallons. The 25 watts was recommended for up to 5 gallons and I wanted to make sure I wasn't over the limit of the heater. I turned the heater all the way on and it can handle a miserable 74 degrees in a 68 degree room. UPDATE July 2019: I broke the heater described above (my fault) and ordered another one (actually two as I will be installing another 5g aquarium in the near future). Yes, yes, I know, the first heating described above was just about enough. But the larger heater is physically too big to fit in the small space at the back of the aquarium. Well, one of the new heaters is NOT enough to heat a 5 gallon aquarium - it was actually set at 92 degrees! Luckily I didn't have any fish in the tank but I lost a few Cryptocoryne plants. Every day for four days, I turned the knob on top of the heater slightly towards the "-" symbol until it stopped turning "down". The aquarium is stable at 80-81 degrees. I would have preferred 78 but maybe it will at least keep the tank warm enough in the winter months when my house is 6 or 8 degrees cooler. I will test another heater in a bucket of water during the warranty period and see what it can/can't do. Conclusion: These are cheap and not permanent heaters. Don't use them for expensive fish.