Water changes: frequency and amount

Water changes are the single most important routine maintenance task. Without them, nitrates accumulate, hardness drops, micronutrients deplete. Without them, biological balance collapses in 1–3 months.

The basic schedule

Standard for a community tropical tank: 25–30 % of the water once a week. That's enough for most situations. Bigger changes aren't harmful but don't deliver proportionally more benefit.

When to change more often

• Tank overstocked with fish — 50 % once a week or 25 % twice a week.

• Discus species tank — 30 % every other day or 50 % once a week.

• After medicating — 50 % several times in a row to remove the drug.

• Nitrate test shows >40 mg/L — increase the change volume.

When to change less often

• Lightly stocked planted tank — 25 % every 2 weeks.

• Walstad biotope (densely planted, few fish) — once a month.

How to prepare the water

Tap water contains chlorine/chloramine and sometimes heavy metals. Preparation options:

• Let it stand 24–48 h in an open container — free chlorine evaporates. Chloramine does not.

• Use an aquarium conditioner (Seachem Prime, Tetra AquaSafe) — instant neutralization.

• For demanding species — RO water with minerals added back.

Water-change temperature

Old water and new water — temperature difference no more than 2 °C. Cold water stresses fish and can trigger ich.

Gravel siphoning

Every 2–4 weeks, vacuum the substrate to pull out accumulated detritus. No more than 1/3 of the area at a time, to preserve the bacterial colony.

What not to do

• Don't change 100 % of the water — it wipes out the biobalance. 80 % is a last-resort emergency only.

• Don't rinse the filter under the tap — chlorine kills bacteria. Rinse only in aquarium water.

• Don't change water more than twice a day — it stresses the fish.

• Don't dose the conditioner "by eye" — too little leaves chlorine intact; in rare cases overdose can also be harmful.

Don't "clean" the aquarium — maintain it. The goal is stability, not shine. The mature biofilm on glass and substrate is health, not dirt.