There were a few things I knew I wanted in a small aquarium heater:

A good rule of thumb to determine what size heater you may need for your small aquarium is:
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If you keep tropical fish in a small tank or , you would need . This 7.5 watt heater will keep the water temperature inside the aquarium warm. Once you put the heater inside the fish tank, it won’t drastically change the water temperature; it will only increase by few degrees at a time so your fish can adjust.
The perfect size heater for MiniBows and other small aquariums.
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When you do go to plug in and turn on the preset heater, you should really have a secondary thermometer on hand. While the temperature gauge built into most of these heaters work fairly accurately, it’s good to have that second thermometer to verify that the temperature is actually what the heater says it is. When making adjustments to the temperature of the aquarium, please make the changes in small increments rather than all at once. Also note that the settings on the knob should be used as only a rough guide to show you in which direction the heater is being adjusted. You shouldn’t always trust what is factory imprinted on the control knob as being what the temperature will end up as. Does anyone know of or have a suggestion as to where I may find a decent heater for a small 2.5 gallon aquarium?
Photo provided by FlickrDeep Blue Professional is literally pro at manufacturing aquarium heaters for smaller tanks like that of bettas.
Photo provided by FlickrMini Heaters for Small Aquariums - The Spruce
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eLive's air operated Betta Filters are designed to filter water in small bowls and aquariums. They improve the water quality, clarity and ultimately provide a healthier environment for your betta fish. This filter comes with a stump that helps to hide the filter.Actually, there is no harm at all in using a large heater in a small tank. Modern aquarium heaters are equipped with a thermostat which turns the heater on and off when the tank water varies from the desired temperature. Using a large heater will result in the heater coming on less often. That will be the only effect this will have."Sure. Most aquarists use heaters that are supposed to be geared towards larger aquariums on smaller sized aquariums to speed up heating and to help keep the temperature constant for longer periods of time. It shouldn't do any harm at all unless you used a heater way out of range like a 500W heater on a 10G. Similarly how most of use larger filters on smaller tanks. HTH.This is a fully submersible aquarium heater known for its durability and one can count on it to last for several years. It features a PTC heating element as well as a shatterproof glass, which comes in handy when heating small fish tanks from about 5 - 10 gallons. The PTC patented heating system allows for a system that guarantees safety. We liked that you can fully submerge the heater, so you could lie it down on the bottom of the tank out of sight.One gallon tanks can be difficult to heat. The small water volume leaves it open to severe fluctuations and most aquarium aren’t made for such small containers. If your room is quite cool you may be able to use the . At just 7.5 watts it is one of the only water heaters made for tiny aquariums. It is important to note that these heaters do not have an automatic shut-off when they reach the desired like most aquarium heaters have (for larger tanks). This means you could easily overheat your water posing a danger to your fish. If you choose to go with a mini-heater you should absolutely have a thermometer in the tank and you should check it frequently. On warmer days you may need to unplug the heater completely.HI,
I have a 1.3 gallon (5L) tank, don’t have a betta yet and i live in New Zealand.
I decided to get my tank set up first and getting it relatively stable before even thinking about introducing a fish. I started by buying a 10W one of those shatterproof heaters and found it would not heat my tank at night above 16 Celcius (66 ish Farenheit), after this I replaced it with a 25W (same type of heater) which would not heat above 22 (72 Farenheit), exasperated i bought an elite 50W heater which for the last three days has kept my aquarium at a constant 26-27 Celcius (79-81 farenheit).
I am confused as to how it has taken this much Wattage to heat a small tank and whether I should trust this heater to keep my aquarium constant for my future pet.