Install the filter in the correct location before adding water
Most aquarium filters mount on the back or side of the tank, or sit inside it, depending on the type. Read your filter's manual first — it will tell you exactly where the intake tube, outlet, and motor housing need to go. The intake tube draws water from the tank, so place it away from the outlet to avoid recycling the same water without filtering it. Position the outlet so water flows across the full width of the tank rather than in one corner.
If you have a hang-on-back filter, clip it to the rim of the tank before you fill it. If you have a canister filter, place the canister on the floor or in a cabinet below the tank — the intake and outlet tubes run to the tank, but the motor stays outside. An internal filter or sponge filter sits fully inside the tank, so position it where it will not block your view or interfere with plants or decorations.
Key Takeaways
- Place the intake tube away from the outlet so water circulates through the entire tank instead of short-circuiting.
- Install the filter before filling the tank to avoid spilling water and to make sure all parts fit properly.
- Prime the filter by filling the intake tube with water before turning it on, or it will suck air and lose suction.
- Run the filter continuously — turning it off and on damages the beneficial bacteria that live in the media and keep the tank safe.
- Clean the filter media every two to four weeks by rinsing it in old tank water, never tap water, to preserve the bacteria.
Attach the intake and outlet tubes to the filter head
Most filters come with plastic tubing that slides onto barbed fittings. Push the intake tube onto the intake fitting and the outlet tube onto the outlet fitting — they should fit snugly. If the tubes are stiff and hard to push on, run them under warm water for a few seconds to soften them. Do not use force; if a tube will not slide on, check that you are using the correct size.
Some filters use quick-disconnect couplers instead of barbed fittings. These click together and apart without tools. Line up the male and female ends and push until you hear or feel a click. Test the connection by gently tugging — it should not come loose.
Place the intake tube in the tank and find the outlet
The intake tube draws water from the tank, so it needs to sit in the water column — usually 1 to 2 inches below the surface. Use a suction cup or clip (often included with the filter) to hold the intake tube in place. Make sure the intake opening is not pressed against the glass or buried in gravel, or it will clog when ready.
The outlet tube directs filtered water back into the tank. Point it slightly downward and toward the center of the tank so the water spreads out. If the outlet is too close to the surface, it will create excessive splashing and noise. If it points straight down, it may stir up debris from the bottom. Adjust the angle until the water flow looks smooth and reaches all parts of the tank.
Fill the filter media and prime the system
Before you turn on the filter, you need to fill it with the correct media. Most filters use filter floss (a coarse layer that traps large particles), activated carbon (which removes odors and discoloration), and biological media (porous material where beneficial bacteria live). Your filter manual will show the order and how much of each to use.
Once the media is in place, fill the intake tube with water using a cup or siphon. This step, called priming, removes air from the tube so the filter can draw water smoothly. If you skip this, the filter will suck air, lose suction, and may not start. Pour water into the intake tube until it is completely full, then turn on the filter. Water should flow out of the outlet within a few seconds.
Turn on the filter and check for leaks
Plug in the filter and watch it for the first few minutes. Water should flow steadily from the outlet. If water is spraying from the intake or outlet connections, turn off the filter and tighten the tube fittings. If water is leaking from the filter body itself, turn it off when ready — this usually means a seal is damaged and the filter needs to be replaced or repaired.
Listen for unusual sounds. A quiet hum or gentle gurgling is normal. A loud grinding noise, rattling, or squealing means something is wrong — the impeller (the spinning part that moves water) may be jammed, or the motor may be failing. Turn off the filter and check that nothing is blocking the intake tube.
Run the filter continuously and establish the bacterial colony
Leave the filter running 24 hours a day, 7 days a week. The filter media is home to beneficial bacteria that break down fish waste and keep the water safe. These bacteria die if the filter is off for more than a few hours. Turning the filter on and off repeatedly damages the colony and makes the tank unsafe for fish.
During the first two to four weeks, the bacterial colony is still growing. This period is called the nitrogen cycle or cycling. Test the water every few days using a kit that measures ammonia, nitrite, and nitrate. Once ammonia and nitrite both read zero and nitrate is present, the tank is ready for fish. Do not add fish before this happens, or they will be poisoned by ammonia.
Clean the filter media without killing the bacteria
Every two to four weeks, rinse the filter media to remove trapped debris. This keeps water flowing smoothly and prevents the filter from clogging. The key rule: rinse the media in old tank water, never in tap water. Tap water contains chlorine and chloramine, which kill the beneficial bacteria you spent weeks growing.
During a water change, collect some of the water you are removing in a bucket. Turn off the filter, open it, and rinse each piece of media in that bucket water. Squeeze the media gently — do not scrub or wring it. Rinse until the water runs clear, then put the media back in the filter. If the filter has a sponge, squeeze it in the bucket water until it stops releasing debris. Turn the filter back on and you are done.
Replace the activated carbon every month, because it becomes saturated and stops working. Replace the filter floss every two to three months or whenever it looks clogged. The biological media can last for years if you rinse it gently in tank water.
Frequently Asked Questions
Can I turn off the filter at night?
No. The beneficial bacteria in the filter media need a constant supply of water to survive. Turning the filter off for even a few hours kills bacteria and destabilizes the tank. If the filter is too loud, consider a quieter model or move the tank to a different room, but do not turn it off.
What if water is leaking from the filter?
Turn off the filter when ready. Check that all tube connections are pushed on firmly and that the filter body has no cracks. If the leak is from a connection, remove the tube, dry the fitting, and push the tube back on. If the leak is from the filter body itself, the seal is broken and the filter must be replaced.
How do I know if the filter is working?
You should see steady water flow from the outlet tube. The intake tube should have a gentle suction — if you hold your hand near it, you should feel water being pulled in. If there is no flow or very weak flow, the intake may be clogged. Turn off the filter and check the intake tube for debris or algae buildup.
Do I need to replace the filter media all at once?
No. Replace only the parts that are clogged or saturated — usually the floss and carbon. Keep the biological media in the filter as long as it is not falling apart. If you replace all the media at once, you lose the entire bacterial colony and have to cycle the tank again.
What size filter do I need?
The filter should turn over the entire tank volume at least four times per hour. For a 20-gallon tank, you need a filter rated for at least 80 gallons per hour. Check the filter box or manual for the flow rate. A stronger filter is better than a weaker one — it keeps the water cleaner and the fish healthier.