The extra heater I have is the Fluval M 100W heater

That's very good to know Sky. I was afraid of the 25 watt getting the water to warm. Don't want to fry the poor fish.
Guess I'm off to the store,lol.
Chris,so far that's about right. I have yet to see a decent heater for these smaller tanks( Someone is going to make a killing in that untapped market). I did however purchase a Jr. Heater from Walmart that's said to be for a 2-5 gallon tank(7.5 watts). I am just a bit concerned due to the fact that it has no thermostat on it. I have a small tank in a room that tends to be a bit cooler than the rest of the house in the winter and I don't want the fish to be subjected to water so cool that the poor thing would freeze.
Guess I'll get a 25 watt and see how that does for me.
Thanks all.

For a 30 gallons capacity fish tanks a heater with wattage of 100-150 watts will be just fine.

This entry is a bit of a cheat as it is a 6 gallon fish tank instead of a 5 gallon. Despite the extra space, this fish aquarium is a perfect fit for this list. It measures approximately 16.9 (L) x 10.2 (W) x 14.5 (H) inches and weighs about 23 pounds, and is an absolute breeze to assemble. It is a six sided, encased tank with a functional column down the middle that is beyond gorgeous – it looks like it travelled back from the future to show us how far aquariums have advanced. The idea of this design is to give you a near 360-degree view, achieved by covering up the three-stage filtration system (mechanical, chemical and biological) behind an architectural structure.

Aquarium & Fish Tank Heaters, Chillers & Thermometers | Petco

For a 1 gallons fish tank a heater with wattage of 50 watts will be just fine. These fit into any tank with a capacity from 13 gallons (50 watt) all the way up to 75 gallons (300 watt). It has an easy to manipulate and use temperature setting and is fully submersible in the tank. It’s good for a small fish tank heater or a large submersible aquarium heater depending on the size you choose.

Recommendations for a filter and heater for a 5 gallon tank

The rule of thumb here is 3 to 5 watts of heat per gallon of water in the tank. For example, if you have a large, 10 gallon fish tank then you’re going to need something that outputs 30 to 50 watts of heat. Keeping the heat output in that range will keep the fish happy and healthy. In this buyers guide we will cover multiple brands of aquarium heaters that each offer a number of different wattages. Pay attention to the wattage and the size of your tank no matter which brand you choose. The wattage ratings range from 25 watts all the way up to 400 watts of the best aquarium heaters.

I've got a single male Betta in a 5 gallon tank

I’ve had my guppy tank for a couple months now. I’ve successfully gotten by with NO heater for my aquarium. What water temperature do guppy fish need? I didn’t think they needed heated water but I could be wrong. Tank details: 10 gallon aquarium with 5 guppies. They seem somewhat different lately.. Maybe I’m over thinking this. Could use advice.:
Guppies are from tropical climates so your tank needs to be of tropical temperatures. You need a temp of 75-80 degrees Fahrenheit. It’s very important to have the correct water temperature for tropical fish. You may be able to hold down that water temperature if you live somewhere with warm weather. If you need to buy a heater, figure on needing 5 watts for every 1 gallon of water in your aquarium. You said your tank is 5 gal so that means you need a 25 watt aquarium heater.