Changing the water in a fish tank seems simple until you see a fish gasping at the surface hours later. Most problems don’t come from cleaning too little, but from cleaning badly. These are the mistakes that happen over and over again, with an explanation of why they occur and how to correct them.
Mistake 1: emptying all the water and washing the filter thoroughly
It’s the most destructive mistake. The filter isn’t just a sponge that traps dirt: it houses the nitrifying bacteria that transform the ammonia from waste into nitrate. If you wash it with tap water and leave it spotless, you eliminate that colony and the aquarium enters a toxic ammonia cycle for days.
How to do it right
- Change between 20% and 30% of the volume, not 100%. In small, heavily stocked aquariums it can go up to 50%, but never a full emptying except in a health emergency.
- Rinse the filter material only in water taken from the aquarium itself, never under the tap.
- Alternate cleaning the sponge and the biological filter media in different weeks so you don’t leave the system without bacterial load.
Mistake 2: cleaning the glass with kitchen products
A case that is seen frequently: someone uses the living room glass cleaner to remove limescale stains and the spray ends up inside the water. Residues of ammonia, alcohol or surfactants are lethal to the gills and skin of fish. The same happens with the dish sponge if it still holds soap.
The safe alternative is a new sponge reserved only for the aquarium or a blade scraper for calcareous deposits. If you need to disinfect decoration, use very hot water and let it dry in the sun; the chlorine evaporates.
Mistake 3: siphoning the substrate over the surface, without depth
Passing the siphon grazing the gravel doesn’t extract the organic remains that accumulate between the stones. Over time, that decomposing matter releases phosphates and feeds algae. But digging too much into fine sand substrates or in planted aquariums also creates problems: roots are disturbed and pockets of hydrogen sulfide are released.
Nuances that matter
In coarse gravel it’s best to push the tube down to the bottom and lift it slightly. In sand, siphoning should be superficial and with the bell tilted. In aquariums with root plants, clean only the clear areas. This doesn’t work the same in all setups: a bare breeding tank is cleaned differently, with direct siphoning and more frequent changes.
Mistake 4: changing the water without controlling the temperature or the chlorine
Pouring lukewarm tap water “by eye” causes thermal shocks of several degrees that stress the fish. In addition, chlorine and chloramine must be neutralized with conditioner before entering the tank; letting the water sit for 24 hours eliminates chlorine, but not the chloramine used by many water treatment plants.
- Adjust the temperature of the new water to that of the aquarium with a thermometer, not by touch.
- Dose the conditioner for the total volume of the container you are preparing, not for that of the aquarium.
- Pour the new water slowly, over a plate or your hand, so as not to disturb the substrate.
Mistake 5: cleaning everything on the same day
Siphoning, changing the filter, scraping glass and washing decoration in a single session leaves the aquarium without defenses. According to documentation from aquarium associations, the sensible routine spreads out the tasks: water change and siphoning weekly or fortnightly, rinsing the sponge every two or three weeks, checking the biological material every month or a month and a half.
A detail that is only learned through practice: after a correct cleaning, fish usually appear active within a few hours. If the next day they are apathetic or with their fins clamped, check whether you have moved too much water or filtered excessively. Cleaning a fish tank isn’t about leaving the glass sterile, but about maintaining a stable balance. No routine guarantees zero problems: water quality also depends on the fish load, feeding and filtration, and no method replaces daily observation of the aquarium.