Skip to content
Fishician®

Source water: the input you did not choose but still have to manage

Every water change and top-off pours something into the tank, and it can change without notice. Know what's in it.

2 min readUpdated September 25, 2026

About this guide

Water quality

By Fishician

Published September 8, 2026

How we source
On this page

The short answer

  • Tap water changes. Treatment varies by season and maintenance. A problem right after a water change points at the new water first.
  • Chloramine doesn't gas off. Unlike chlorine, it stays. Use a conditioner that says it handles chloramine.
  • Both harm the filter bacteria. Never rinse filter media under untreated tap water.
  • Softened water is not purified water. Many household softeners replace calcium and magnesium with sodium or potassium; they do not remove all dissolved substances.
  • Prepare purified water for its purpose. New freshwater usually needs suitable minerals; marine water needs salt mix. Evaporation top-offs generally need neither.
A compact reverse-osmosis unit with tubing and filter housings
A reverse-osmosis unit used for aquarium water preparation. The final water still needs to match the needs of its inhabitants.Photo: Auqa osmosis — Andy king50 · Public domain. Resized for display.
Four kinds of source water
City: chlorine
  • Leaves standing, agitated water over time
  • Still use a conditioner
City: chloramine
  • Stable; doesn't gas off
  • Breaking it releases ammonia
Well
  • Can carry gases, iron, sulfide or nitrate
  • Test before relying on it
RO / DI
  • A blank slate
  • Prepare for a water change; top-offs differ
  • Check output on a schedule

Know your supply

  • Is it chlorinated or chloraminated? What are its hardness, alkalinity and nitrate?
  • Read your utility's published water-quality report; it tells you more than one home test.
  • Very low alkalinity means little buffering, one reason pH drifts down between changes.
  • Nitrate or phosphate in the supply feeds algae with every change.

Draw it right

  • Use a cold line run briefly first; standing pipe water can pick up metals.
  • Never use hot-water lines. Heat the water separately.

Prepare it in a bucket, not the tank

  1. Condition, following the current label for your exact product.
  2. Marine: dissolve salt fully and measure salinity with maintained equipment.
  3. Match temperature and chemistry to the tank.
  4. Catch mistakes while they only affect a bucket.

Top-off vs water change

  • Evaporation leaves salt and most minerals behind. RO/DI water is commonly used for top-offs without adding salt or remineralizer.
  • A water change replaces both. Mixing them up slowly shifts salinity and hardness. See water changes.

Re-check when

  • An unexplained problem follows a water change.
  • The utility does maintenance, the season changes, or well conditions change.
  • Purification membranes or resin are due for service; they fade gradually, not visibly.

When purified water helps

  • Your tap water is unsuitable for the fish you keep.
  • You need chemistry the tap can't provide.
  • A marine or specialized system needs control over what goes in.
  • For water changes, match the required mineral content or mix marine salt according to the product instructions. Do not apply that recipe to evaporation top-offs.

Sources

Each source is listed with the part of this guide it supports.