One of the most common questions among Discus keepers is:

Should you use reverse osmosis (RO) water or tap water to keep Discus?

For many years, Discus were considered extremely demanding fish that required very soft and acidic water. However, today's reality is quite different.

The choice between RO water and tap water mainly depends on the origin of your Discus, the quality of your local water, and your goals.

Why Do Wild Discus Require Very Soft Water?

In their natural habitat, wild Discus live mainly in the Amazon Basin.

The water is generally:

• Very low in minerals;
• Slightly acidic (pH 5.0 to 6.5);
• Very low in carbonate hardness (KH);
• Extremely stable.

Wild Discus are naturally adapted to these water conditions.

For their long-term maintenance, and especially for breeding, using remineralized RO water is often essential.

Tank-Bred Discus Are Different

For several decades, most Discus available from European and Asian breeders have been bred in captivity.

Generation after generation, these fish have adapted to a much wider range of water conditions.

They can thrive in tap water, provided that the water quality is suitable.

At LB Discus, every fish is raised from birth in water similar to the tap water found in many regions of France.

This makes acclimation much easier for hobbyists.

The Benefits of Tap Water

In many situations, tap water is the best choice.

It offers several advantages:

• Excellent parameter stability;
• Easy to use without specialized equipment;
• Very low operating cost;
• Fewer water preparation steps;
• Lower risk of remineralization errors.

Stable water is often far more beneficial than theoretically perfect water with constantly changing parameters.

When Should You Use RO Water?

RO water becomes useful when your tap water is not suitable.

For example, if your water has:

• Very high GH (above approximately 15–18°dGH);
• High KH;
• High nitrate levels;
• Excessive conductivity;
• Certain undesirable contaminants.

In these situations, RO water provides a nearly pure starting point.

It must then be remineralized to provide the essential minerals required by Discus.

Using pure RO water without remineralization can lead to serious health problems.

Can You Mix RO Water and Tap Water?

Yes.

In fact, this is a very common practice.

Mixing both water sources allows you to achieve the desired hardness while maintaining good stability.

The mixing ratio depends entirely on the characteristics of your local tap water.

Since water quality varies from one region to another, it is recommended to regularly test:

• GH;
• KH;
• pH;
• Nitrate levels;
• Conductivity.

Stability Is More Important Than Perfection

For Discus, stable water conditions are often more important than achieving "perfect" numbers.

Sudden fluctuations in pH, hardness, or conductivity create significantly more stress than water that is slightly harder but remains stable over time.

Consistency is one of the keys to keeping healthy Discus.

What Water Does LB Discus Use?

All Discus offered by LB Discus are raised in water adapted to typical French tap water.

The objective is simple:

to allow customers to acclimate their fish easily without needing a complex RO water system.

Of course, tap water varies depending on where you live.

If your water is particularly hard or contains high nitrate levels, mixing it with RO water may be recommended.

Our Advice

Before making any changes to your water, test its parameters.

If your tap water has reasonable values and remains stable over time, it will generally be perfectly suitable for tank-bred Discus.

Investing in an RO unit only becomes truly necessary when your local water quality requires it or when breeding wild Discus.

Conclusion

There is no universal answer.

For wild Discus, RO water is often essential.

For properly acclimated tank-bred Discus, good-quality tap water is generally the simplest, most economical, and most reliable solution.

In the end, stable and well-maintained water will always be more important than chasing theoretically perfect water parameters.

Article written by Franck Le Bozec – Discus breeder in Brittany

Founder of LB Discus, a breeding facility specialized in French-bred discus reproduction.