Review on 🐠 Aqueon Mini Submersible Aquatic Flat Heater - Black, 5 Watts | Ideal for Up to 2.5 Gallon Tanks by Angel Yergeau

This is not an adjustable heater. Other than that, it fits perfectly in my 2.5g beta tank. I throw it in, plug it in and it keeps water at around 76-78 degrees. Sometimes less, sometimes more. My beta doesn't need a fixed temperature, he's happy as long as he's not cold. If I plan on replacing the whole bowl, I put the heater in the jug of clean water to heat it up first to avoid shock. This takes some time, so I just let it sit until the thermometer registers an acceptable temperature. The heater doesn't have this, so you'll need to buy your own thermometer if you want to check the temperature.