# Water Quality During Fish Treatment: What to Test and Change

> During treatment, test ammonia and nitrite daily and keep both at zero, keep temperature and pH stable, and change water as the medication label allows, re-dosing for the volume replaced. Remove activated carbon, keep aeration high, and avoid changes that stress a fish already fighting infection.

Source: https://petremedyusa.com/guides/water-quality-during-treatment/ · Updated 2026-09-30 · PetRemedyUSA

## Why water quality is half of the treatment

A sick fish is under stress, and poor water adds more. Many bacterial diseases begin or worsen because of high ammonia or nitrite. Even the best medication cannot outwork a toxic environment.

## Parameters to test

| Parameter | Target | Why it matters |
| --- | --- | --- |
| Ammonia | 0 ppm | Toxic; burns gills and skin |
| Nitrite | 0 ppm | Prevents oxygen transport in blood |
| Nitrate | As low as possible, ideally under 20 ppm | Chronic stress at high levels |
| Temperature | Stable and within the species' range | Swings weaken immunity |
| pH | Stable | Sudden shifts stress fish and can change ammonia toxicity |

## Test daily during treatment

Antibiotics can reduce beneficial bacteria, which may lead to ammonia spikes. A daily check catches problems early, and you can respond with a water change.

## Managing water changes

- Follow the medication label for when to change water and how much to re-dose.
- Match temperature and use dechlorinated water.
- Replace medication in proportion to the water removed.

## What to remove or avoid

- Activated carbon and some filter media that absorb medication
- UV sterilizers and ozone during treatment, unless the label says otherwise
- New fish, which add stress and risk

## Keep oxygen high

Stressed or sick fish need more oxygen. An air stone or sponge filter increases surface agitation. Warmer water holds less oxygen, so avoid raising temperature without adding aeration.

> **Tip:** Write down every test result. A trend, such as ammonia creeping up, is more useful than a single reading.

## Frequently asked questions

### How often should I test water while treating fish?

Daily is ideal, and more often if ammonia or nitrite is present. Antibiotics can disturb the filter bacteria that keep these at zero.

### Should I do water changes during treatment?

Yes, as the medication label directs. Clean water helps recovery, and you replace medication for the volume removed.

### Why remove carbon during medication?

Activated carbon absorbs many drugs and can strip them from the water before they work.