The mistakes that ruin your aquarium water (and how to avoid them)
Most problems in a home aquarium don’t come from visible dirt, but from poorly executed cleaning. A water change that is too aggressive can wipe out the filter’s bacterial colony and cause an ammonia spike that kills fish within 24-48 hours. According to technical documentation from water conditioner manufacturers, the nitrogen cycle takes 4 to 6 weeks to stabilize, and any abrupt intervention partially restarts it.
Mistake 1: emptying the entire tank and washing everything with soap
It’s the most common mistake among beginners. Emptying the entire aquarium, scrubbing the walls with detergent, and boiling the gravel destroys the biofilm where nitrifying bacteria live. The result: crystal-clear water for two days and then a die-off.
What to do instead
Change only 20% to 30% of the volume each week or every two weeks, depending on the fish load. Vacuum the bottom with a siphon without disturbing all the gravel. Never wash the biological filter media under the tap: rinse it in a bucket with water from the aquarium itself to preserve the colony.
Mistake 2: cleaning the filter on the same day you change the water
This is a mistake you only catch with experience. If you combine the partial water change with filter maintenance, you suddenly reduce the bacterial population and its capacity to process waste. I’ve seen it in 60-liter aquariums with schooling fish: after that double cleaning, the water clouds up and measurable ammonia shows up on a drop test.
Alternate the tasks: one week a water change, the next a gentle cleaning of the mechanical prefilter. And wait at least 48 hours between the two.
Mistake 3: using a net or sponge with soap residue
Any trace of detergent, disinfectant, or glass cleaner is toxic to fish. Keep tools exclusively for the aquarium and rinse them only with hot water. For algae on the glass, a silicone scraper or a new, untreated sponge is enough; diluted vinegar works for mineral deposits on the glass, but it must be rinsed thoroughly and never applied with fish in the tank.
Mistake 4: changing 100% of the water ‘because it’s very dirty’
A neglected aquarium with high nitrates isn’t fixed by a full drain. The sudden change in parameters (pH, hardness, temperature) stresses the fish and can trigger disease. The right approach is to do more frequent partial changes over two or three weeks until values normalize. According to aquarium hobby guides from industry associations, dropping nitrates suddenly below 20 mg/l in a single day is more dangerous than keeping them temporarily high.
Mistake 5: ignoring the temperature and chlorine of the new water
Tap water contains chlorine and chloramines that damage gills. Use a specific conditioner or let the water sit for 24-48 hours in an open container (this doesn’t remove chloramines, only chlorine). Match the temperature before pouring: a difference of more than 2 ºC causes thermal stress. Pour the new water slowly, against a plate or the wall, so you don’t disturb the substrate.
Mistake 6: over-cleaning the decorative substrate
Stones and driftwood don’t need to be spotless. A certain layer of algae and biofilm is part of the ecosystem. If you scour all the gravel every week, you remove food for snails and microfauna. Vacuum only the areas with visible debris buildup.
An important nuance
Not all aquariums are cleaned the same way: a planted tank with CO2 and nutrient-rich substrate requires less vacuuming of the bottom so as not to break up the substrate, while a cichlid aquarium needs more intensive siphoning. And if your filter is a model with a biological basket, check the manufacturer’s manual: some recommend changing the carbon every month, others advise against using it permanently. Proper cleaning depends on the type of aquarium, not on a one-size-fits-all recipe.