Aquarium Filter and Heater Guide for Beginners
6 minute read · Updated November 4, 2026
Edited by Ivan F.
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Quick answer
A sponge filter is the gentle, low-cost choice for small tanks and long fins. A hang-on-back filter is simple to service and traps more dirt. A canister is quiet and powerful, but it costs more and takes more work. For heaters, the common rule is about 5 watts per gallon, adjusted for a cold room.
Filters compared
A filter removes waste and gives the helpful bacteria a place to live. Our cycling guide covers those bacteria.
| Sponge | Hang-on-back | Canister | |
|---|---|---|---|
| Where it sits | Inside the tank | On the back rim | Outside, with hoses |
| Runs on | An air pump | A small water pump | A pump in the canister |
| Flow | Gentle | Stronger, and some models adjust | Powerful, and may need slowing |
| Strength | Cheap, hard to break, gentle on fry and shrimp | Better at trapping dirt, easy to service | Powerful and quiet |
| Drawback | Takes up space in the tank | Can burn out if it runs dry, waterfall noise | Costs more, and cleaning means taking it apart, with a flood risk |
| Watch for | Needs cleaning when flow drops | The intake can pull in sand, so add a sponge | Leaks |
Sources do not all agree. The retailer behind most of this table is positive about hang-on-back filters. One keeper site dislikes them, saying many hold too little media for the bacteria, and suggests adding foam inserts. It also says hang-on-back and canister filters last about five years, and that leaks and blown motors are common. Both are hobby views, not independent tests.
Does every tank need one?
A welfare charity says no fish should be kept without a well set up filter, and a veterinary hospital group says all tanks need one. A veterinary manual says very small tanks may manage without, with more frequent, larger water changes. We recommend a filter for any first tank.
Looking after it
- Service power filters and air pumps every 1 to 3 months, says a veterinary manual, and check flow every 2 weeks.
- The same manual says to change replaceable cartridges once a month, or sooner if clogged. Many hanging filters also hold a separate medium for the bacteria, so keep that in place.
- Rinse sponges and media in old tank water, never under the tap. Tap water kills the bacteria.
Flow and long fins
Strong current can whip fish around the tank, and fish may hide or get sick from the ongoing stress. One retailer says the fish most affected have long, flowing fins or small bodies, or come from slow water, and it names bettas and cherry shrimp. The default filter in an all-in-one kit can be too strong for slower fish.
Signs to watch for: a fish struggling to swim or being pushed around, or one that keeps hiding from the outflow. If you see them, slow the flow:
| Filter | Ways to slow it |
|---|---|
| Sponge | Add a valve to the air line, or use an adjustable pump |
| Hang-on-back | Fit coarse sponge in the waterfall opening, stretch craft mesh across it, or place plants or decorations in front of it |
| Canister | Aim the outflow at the back wall or surface, add a sponge to the outlet, or use a spray bar |
If fish still struggle, the retailer says to move down a size. For Bettas, see our Betta page.
Which filter suits which tank
No source we read gives firm cut-offs by tank size, so this table is our advice. It matches our starter kits.
| Tank | Our suggestion | Why |
|---|---|---|
| 5 to 10 gallons | Sponge | Gentle, simple, and safe for shrimp and small fish |
| 20 to 29 gallons | Hang-on-back or large sponge | Easy to service, and better at trapping dirt |
| 40 to 55 gallons | Large hang-on-back or canister | Handles more waste, and a canister holds more media |
Each fish page lists gear for its tank size.
Do you need a heater?
A veterinary manual says a heater is necessary for tropical fish. Goldfish and some other cold-water fish do fine at room temperature. Check the range on your species page.
What size heater?
The old rule of thumb is 5 watts per US gallon, or about 1 watt per liter. One keeper guide says heaters are designed to lift water about 9°F (5°C) above the room, and that a colder room needs more. One hobby site says to use a larger size or a second heater in a cold room or beside an outside wall.
| Tank | Rule of thumb |
|---|---|
| 5 gallons | 25 W |
| 10 gallons | 50 W |
| 20 gallons | 100 W |
| 29 gallons | 145 W |
| 40 gallons | 200 W |
| 55 gallons | 275 W |
Treat the rule as a starting point. One keeper guide warns that a heater may not deliver its rated power. Check your tank with a thermometer, and size up if it cannot hold temperature.
Why not just buy the biggest?
A heater can fail by staying on, and it can then overheat the tank. One hobby site warns this can kill your fish. One keeper guide suggests two heaters of half the required power each, so a stuck heater has less power to do harm. If one stops, the other keeps some warmth in the water.
Heater safety
Read the instructions for your heater
Some are not designed to be fully under water, and some do not switch themselves off when lifted out.Place it well
One keeper guide suggests low in the tank at an angle, and one hobby site warns never to rest it against the gravel, because the glass may crack.Use a separate thermometer
A veterinary manual says to monitor temperature independently of the heater. Heaters and thermostats can fail, so a welfare charity says to check every day.Check your outlets
A veterinary manual asks whether your outlet is grounded and has a GFI (ground-fault interrupter), and suggests a surge protector.Add a drip loop
Let each cord dip below the outlet before it rises to the plug, so water drips off the low point rather than running into the socket. One manufacturer says to always include one.Keep the sun off the tank
Direct sunlight can warm the water, and warmer water holds less oxygen.
Our other guides cover choosing your tank size and bringing new fish home.
Products mentioned in this guide
Each one is a pick on one of our buying pages, which explains the choice. Nobody at AquariaWiki has used or tested these products. The specifications are the makers' own statements, so these are the strongest fits on the facts we could record, not a hands-on verdict.
Picks for the US
Aquarium Co-Op Coarse Sponge Filter (Small) US
The simplest option here: no moving parts, and its sizing starts at this tank size. You also need to buy an air pump, tubing and a check valve, and the maker warns the uplift tube may need trimming in a shallower tank. The maker is the same store whose articles this page cites, so weigh its claims accordingly.
The manufacturer states: Coarse sponge filter with no moving parts, gentle enough for betta fish, baby fry and shrimp. Needs an air pump, airline tubing and a check valve. Small is sized for 10 gallons and up. Shallower tanks may need the uplift tube trimmed.
Why we chose it (Best Filter for a 10-Gallon Tank)
Fluval AquaClear 20 Power Filter US
The strongest fit if you want a hang-on-back: its rating includes this tank, and you can turn the flow down. The maker gives no figure for the low setting, and a keeper site says many hang-on-backs hold too little media. Our advice is to fit a sponge over the intake if you keep shrimp or fry.
The manufacturer states: Hang-on-back for 5 to 20 US gallons, with flow control it says lets you adjust the water flow. Maximum flow of 100 US GPH on its series page. Foam and carbon media included.
Why we chose it (Best Filter for a 10-Gallon Tank)
Fluval T50 Fully Electronic Aquarium Heater US
Its page states a power cut when out of water and a guard against breakage, and its wattage matches the rule of thumb above at this tank size. Check its length against your tank's height. Those features are not protection from a thermostat stuck on, and its page promises none, so you still need a separate thermometer.
The manufacturer states: 50 W, for aquariums up to 15 US gallons. Dial from 72 to 90°F. Automatically cuts power in the absence of water, with a heater guard against breakage. 10.5 inches long.
Why we chose it (Best Heater for a 10-Gallon Aquarium)
Aqueon Pro Adjustable Heater 50 W US
A plastic housing, two stated shut-offs, and the shorter of these two picks. Its rating is more generous than the rule of thumb above, so do not read it as a reason to use one on a larger tank. Its page does not say either shut-off covers a thermostat stuck on, so you still need a separate thermometer.
The manufacturer states: Shatterproof thermal plastic housing. Adjustable 68 to 88°F, thermostat accurate to plus or minus 1 degree. Auto shut-off when it overheats, resetting when it cools, and when the top tilts below the bottom. Recommended for aquariums up to 20 gallons. About 9 inches long. Limited lifetime warranty.
Why we chose it (Best Heater for a 10-Gallon Aquarium)
Picks for Spain
Tetra Brillant Filter Spain
The simplest option here: it runs on air, with no motor in the tank, and its rating covers this tank. You also need to buy an air pump sold for Spanish mains, tubing and a check valve. Its rating runs far above a tank this size, and the maker gives no flow figure.
The manufacturer states: Internal filter driven by air, with a foam cartridge. For aquariums of up to 100 liters. Tetra suggests its APS 100 air pump.
Why we chose it (Best Filter for a 10-Gallon Tank)
Fluval AquaClear 20 Power Filter Spain
The strongest fit if you want a hang-on-back, and its rating includes this tank. Choose one sold for Europe, because this series is also made for US mains. The maker gives no figure for a low setting on the pages we read, which are in German. Our advice is to fit a sponge over the intake if you keep shrimp or fry.
The manufacturer states: AquaClear AC20, for 18 to 76 liters. Pump output of 379 l/h at 230-240 V, 50 Hz.
Maker's page · Sold at Kiwoko · Sold at Tiendanimal
Why we chose it (Best Filter for a 10-Gallon Tank)
Fluval T50 Fully Electronic Aquarium Heater Spain
Its page states a power cut when there is no water and a guard against breakage, and its wattage matches the rule of thumb above at this tank size. Choose one sold for Europe, because this series is also listed for US mains. Those features are not protection from a thermostat stuck on, and its page promises none, so you still need a separate thermometer.
The manufacturer states: 50 W, for aquariums up to 50 liters. Dial from 22 to 32°C. Cuts power when there is no water, with a heater guard. Listed at 230-240 V, 50 Hz.
Maker's page · Sold at zooplus.es
Why we chose it (Best Heater for a 10-Gallon Aquarium)
Eheim Thermocontrol e 50 Heater Spain
The closest stated accuracy here, with a rating centered on this tank size and stated dry-running protection. It is coated in glass, not plastic, so handle it with care. That protection is not a promise about a thermostat stuck on, so you still need a separate thermometer.
The manufacturer states: 50 W, for aquariums of about 25 to 60 liters. Adjustable 20 to 32°C, accuracy plus or minus 0.5°C. Dry-running protection. 233 mm tall. 3-year warranty. Listed at 220 to 240 V.
Maker's page · Sold at Kiwoko · Sold at Tiendanimal
Why we chose it (Best Heater for a 10-Gallon Aquarium)
Frequently asked questions
Does every aquarium need a filter?
A welfare charity and a veterinary hospital group say yes. A veterinary manual says very small tanks may manage without, but only with much more frequent water changes. We recommend a filter for any first tank.
What heater size do I need for a 20-gallon tank?
The common rule of 5 watts per gallon gives 100 watts. It assumes a rise of about 9°F over the room, so a colder room needs more, or a second heater. Always check with a separate thermometer.
Is one big heater or two small heaters safer?
One keeper guide advises two heaters of half the required power, because a single stuck heater then has less power to overheat the tank. A thermometer you check daily is needed either way, since any heater can fail.
What filter is best for a betta?
One retailer calls sponge filters its favorite for gentle flow, and names bettas among the fish that struggle in strong current. If you use a hang-on-back or canister, slow the outflow with a sponge, or aim it at a wall.
Should I unplug the heater during a water change?
Yes, before the water level drops. A heater that is out of the water can overheat, and most are in a glass tube that may shatter. Plug it back in only once the water is back up.