The first time I cleaned a 60-liter fish tank, I lost three neon tetras because I emptied all the water and washed the filter under the tap. That mistake, which seems harmless, eliminates the filter’s bacterial colony and leaves the aquarium cycling all over again. I mention it because almost no one says that cleaning a fish tank is not the same as disinfecting it: the goal is to remove waste, not to sterilize the system.
Before you put your hand in: what to observe for a week
According to documentation from filter manufacturers and aquarists with years of practice, it is advisable to record three things for seven days before touching anything:
- Water cloudiness: if it becomes cloudy within 24 hours, the problem is biological, not aesthetic.
- Dirt spots in the substrate: feces accumulate in areas with less current. That is where you should aim the siphon bell.
- Condition of the plant leaves: brown leaves indicate an excess of dissolved organic matter.
In a 100-liter aquarium with bottom-dwelling fish, I usually do 25-30% water changes every two weeks. In a 20-liter one with a betta, 25% weekly is more realistic. There is no universal figure: it depends on the biological load, not on size.
Real cases: three routines that work in different contexts
1. Cold-water aquarium with goldfish (60 liters)
Goldfish produce a huge amount of waste. I clean with a bell siphon every five days, vacuuming the bottom without disturbing all the substrate. The key: do not empty more than a third of the water. The filter is rinsed only in water from the aquarium itself, never under the tap. If the tap water comes out very cold, I let it sit with dechlorinator for 24 hours beforehand.
2. 30-liter planted nano aquarium
Here aggressive siphoning is counterproductive. I use a thin hose with a gentle flow and only vacuum the visible areas. I remove dead leaves with tweezers. I change 20% weekly. A common mistake in planted tanks: cleaning the glass with abrasive sponges that scratch the acrylic. Better to use a magnet with a plastic blade.
3. Breeding tank with fry
In this case, any siphon with too much suction will suck up the fry. I use an air tube with a nylon stocking over the end to slow the flow. I change 10% daily during the first weeks. There is no shortcut: when breeding, cleaning is constant and gentle.
The real step by step, with the nuances that matter
- Unplug the filter and the heater before putting your hands in. A heater left on out of the water will crack.
- Prepare the replacement water in a clean bucket, for aquarium use only. Add conditioner according to the manufacturer’s dose. Do not use soap or detergent in that bucket.
- Scrape the glass with a specific sponge or a magnetic cleaner. On glass, avoid metal scouring pads.
- Siphon the bottom with the bell, without disturbing all the substrate. Vacuum in a spiral from the center toward the edges.
- Remove the water into a separate container and do not pour it down the sink if you have plants: it works as fertilizer.
- Rinse the filter material only if the flow has dropped a lot, and always in water from the aquarium itself.
- Refill with the prepared water at a similar temperature. Pour over a plate so as not to disturb the substrate.
- Reconnect and wait 15 minutes before checking parameters. Ammonia levels can be temporarily altered.
What does not work (and what no one warns you about)
Emptying the entire fish tank and washing everything with hot water is not cleaning: it is restarting the cycle. Using household cleaning products near the aquarium does not work either; aerosols contaminate the water. And an important nuance: the siphon does not replace the water change, they are complementary.
There are limitations I acknowledge: I do not have data to ensure the optimal frequency in marine aquariums, and in aquariums with shrimp or snails, suction must be minimal or they get damaged. In those cases, I recommend consulting specialized aquarium forums before setting a routine.
Cleaning a fish tank is not a closed procedure: it is a routine that adjusts according to observation. If something goes outside what is expected, the best source is the official documentation from your filter’s manufacturer and the experience of other hobbyists who keep systems similar to yours.