Cover graphic by KnackDaily.
Scrape the algae off the glass, siphon a portion of the water out through a gravel vacuum, and top back up with dechlorinated water matched to the tank temperature. Leave the filter media alone unless it is genuinely clogged, and never take the whole tank apart. A full teardown resets the biology.
The filter is a bacteria farm, not a sponge
Fish produce ammonia, which is toxic to them even in small amounts. In an established aquarium, colonies of nitrifying bacteria convert that ammonia into nitrite and then into far less harmful nitrate, and nearly all of those bacteria live on the surfaces inside the filter. The sponge or ceramic media is not there to strain particles out of the water so much as to give the colony somewhere to grow.
That reframes the whole job. Rinsing filter media under a tap washes chlorinated water through the colony and kills a large part of it, which is why so many tanks go cloudy and lose fish a few days after a thorough cleaning. The tank did not get dirty. It lost its biology and started cycling again from scratch.
The rule that follows is simple. Squeeze the media in a bucket of old tank water when flow slows, replace only one part of the media at a time so the remaining surfaces can reseed the new material, and give the tank a couple of weeks between doing that and any other major disturbance.
Partial changes beat a full teardown
Nitrate builds up in a closed box of water and there is no biological step that removes it, so diluting it is the point of a water change. Doing a slice regularly keeps the chemistry inside a narrow band, and fish handle a narrow band comfortably. Replacing most of the volume in one go swings temperature, hardness and pH at the same moment, and that swing is genuinely stressful.
New water needs two things before it goes in. It needs a dechlorinator, because chloramine in particular does not gas off by standing overnight the way plain chlorine partly does. And it needs to be close to tank temperature, tested against the back of your wrist or a thermometer rather than guessed at the tap.
Working the gravel vacuum
- Unplug the heater and the filter first, so neither runs dry while the level drops.
- Scrape the glass with an algae pad or a blade scraper before you siphon, so the loosened algae goes out with the water.
- Push the wide tube into the gravel and let the flow lift the stones while the debris rides up the hose. Hold it there until the stones tumble clean, then pinch the hose and move on.
- Cover a section, not the whole bed. Doing one part of the substrate per session spreads the disturbance out over several weeks.
- Refill slowly by pouring onto a plate or your hand so the stream does not dig a crater in the substrate.
- Restart the equipment and check that the filter has primed and the heater is submerged.
What keeps a tank stable and what harms it
| Action | Effect on the tank |
|---|---|
| Regular partial water change | Dilutes nitrate while chemistry stays steady |
| Rinsing media in old tank water | Clears clogging without touching the bacteria |
| Vacuuming one section of gravel per visit | Removes waste and leaves colonies intact elsewhere |
| Rinsing media under the tap | Kills the colony and restarts the nitrogen cycle |
| Any soap, even rinsed off | Residue is lethal to fish and impossible to remove fully |
| Full teardown and scrub | Strips the biology and usually triggers a bloom |
| Household spray used near the tank | Aerosol drifts onto the surface and dissolves in |
Algae, light and the food you are adding
Green film on the glass is a symptom rather than a problem in itself. Algae grows where light and nutrients are both plentiful, so a tank sitting in afternoon sun or running its light for most of the day will keep producing it no matter how often the glass gets scraped. Putting the light on a timer and moving the tank out of direct sun does more than scrubbing.
Overfeeding is the other half. Food that is not eaten within a couple of minutes sinks, decomposes and feeds both algae and your ammonia load. Feeding a smaller amount and watching it disappear is the cheapest water quality improvement available. Keep two buckets that never see soap, label them for aquarium use only, and the routine takes about fifteen minutes.
