RO Water for Aquariums: When You Need It and How to Use It

Table of Contents

• What Is Reverse Osmosis?
• When You Actually Need RO Water
• Singapore Tap Water: Do You Even Need RO?
• RO/DI Unit Setup
• Cost in SGD
• Membrane and Filter Replacement
• Waste Water Ratio and Management
• Storage and Handling
• Mixing RO Water With Tap Water
• Frequently Asked Questions

What Is Reverse Osmosis?

Reverse osmosis (RO) is a water purification process that forces water through a semi-permeable membrane under pressure. The membrane’s pores are so fine (typically 0.0001 microns) that they block dissolved minerals, heavy metals, chlorine, chloramine, pesticides, and most other contaminants. What passes through is nearly pure water with a TDS of 0 to 10 ppm.

A standard aquarium RO unit consists of several stages:

1. Sediment pre-filter: Removes particles like sand, rust, and debris that would damage the membrane.
2. Carbon pre-filter: Removes chlorine and chloramine, which degrade RO membranes. This stage is particularly important in Singapore, where PUB uses chloramine for water treatment.
3. RO membrane: The core filtration stage that removes dissolved solids.
4. DI (deionisation) resin (optional): A final polishing stage that reduces TDS to near zero. RO alone typically achieves 90 to 98 percent rejection; adding DI brings this to 99 percent or higher.

For most aquarium purposes, an RO unit without the DI stage is sufficient. The DI stage is primarily useful for hobbyists who need extremely pure water (TDS of 0 to 2 ppm), such as those breeding very sensitive Caridina shrimp varieties.

When You Actually Need RO Water

RO water is not necessary for every aquarium. It adds cost, complexity, and maintenance to your setup. Before investing in an RO system, consider whether your specific situation genuinely requires it.

You Likely Need RO Water If:

• You breed Caridina shrimp: Crystal Red, Crystal Black, Taiwan Bee, and other Caridina varieties require soft, acidic water with precisely controlled GH (4 to 6) and TDS (100 to 150). Achieving and maintaining these parameters consistently is difficult with tap water alone.
• You keep discus: Discus thrive in soft, slightly acidic water with low TDS. While some discus keepers succeed with tap water, breeding discus typically requires the parameter precision that RO water provides.
• Your tap water TDS is very high: In some countries, tap water TDS exceeds 300 to 500 ppm. Starting with RO water and remineralising to the desired level is the only practical approach in these cases.
• Your tap water contains problematic contaminants: In areas with poor water quality, old pipes (lead, copper), or agricultural runoff, RO filtration provides essential purification that a simple dechlorinator cannot.

You Probably Do Not Need RO Water If:

• You keep common tropical community fish (tetras, rasboras, gouramis, corydoras)
• You keep Neocaridina shrimp (Cherry, Blue Dream, etc.)
• Your tap water TDS is below 150 ppm and GH is below 6
• You are not breeding species with specific soft-water requirements

Singapore Tap Water: Do You Even Need RO?

This is the question every Singapore hobbyist should ask before investing in an RO system, and the honest answer for most people is: probably not.

Singapore’s PUB tap water is, by international standards, remarkably clean and quite soft. Typical parameters are:

For the vast majority of tropical fish and Neocaridina shrimp, Singapore tap water is perfectly adequate after dechlorination. The TDS is already low, the GH is soft, and there are no heavy metals or agricultural contaminants to worry about. Adding a dechlorinator that neutralises chloramine is the only treatment most tanks need.

The main group of Singapore hobbyists who genuinely benefit from RO water are Caridina shrimp breeders. Caridina require GH of 4 to 6 with zero KH and a TDS of 100 to 150, parameters that are difficult to achieve by modifying tap water (you cannot easily remove specific minerals from tap water; you can only add them to pure water). For these hobbyists, RO water with controlled remineralisation is the standard and best approach.

RO/DI Unit Setup

Setting up an RO unit in a Singapore HDB flat or condo is straightforward, though it requires some planning around plumbing and space.

Typical Setup

• Location: Most hobbyists connect the RO unit to the kitchen tap or a bathroom tap via a diverter valve. This allows you to switch between normal tap water flow and RO production without permanent plumbing modifications. For HDB flats, the kitchen tap is usually the most convenient connection point.
• Mounting: Compact RO units can sit under the kitchen sink, in a cabinet, or on a shelf near the tap. Some hobbyists mount the unit on a wall for a more permanent installation.
• Hoses: You need an input hose from the tap to the unit, an output hose from the unit to your storage container, and a waste hose directed to the drain or a waste water collection bucket.

Plumbing Considerations for HDB/Condo

If you are renting, a diverter valve that screws onto the existing tap is the best option, as it requires no permanent modifications. If you own the flat, a dedicated T-connector with a ball valve under the sink is a cleaner, more permanent solution. In either case, no BTO renovation work is required; a basic RO installation takes 30 minutes with hand tools.

Cost in SGD

Here is a realistic breakdown of what an RO system costs in Singapore.

The initial investment is modest, and ongoing costs are low. The biggest hidden cost is actually water: RO systems waste a significant amount of water, which adds to your PUB utility bill.

Membrane and Filter Replacement

Maintaining your RO unit is simple but cannot be neglected. A poorly maintained unit produces water that is not truly pure, defeating the entire purpose.

Replacement Schedule

• Sediment pre-filter: Replace every six months. This filter catches large particles and protects the carbon filter and membrane. A clogged sediment filter reduces output flow rate and puts stress on the membrane.
• Carbon pre-filter: Replace every six months. This is critical in Singapore, as the carbon filter removes chloramine from PUB water. If the carbon is exhausted, chloramine passes through and degrades the RO membrane, drastically shortening its lifespan.
• RO membrane: Replace every two to three years under normal use. Test your product water TDS regularly; when the membrane’s rejection rate drops below 90 percent (i.e., output TDS is consistently above 10 percent of input TDS), it is time for a new membrane.
• DI resin: Replace when the resin changes colour (most resins change from one colour to another as they exhaust) or when output TDS rises above 0 to 2 ppm.

Monitoring Membrane Health

Keep a TDS meter on hand and test both your input (tap) water and output (RO) water regularly. A healthy membrane produces water with TDS at 2 to 5 percent of the input TDS. If your tap water is 80 ppm TDS, your RO output should be 2 to 4 ppm. If output TDS rises significantly, the membrane is failing or the pre-filters are exhausted and allowing chloramine to damage the membrane.

Waste Water Ratio and Management

RO systems produce waste water (also called reject water or brine) as a byproduct of the filtration process. This is water that did not pass through the membrane and contains the concentrated dissolved solids that were removed.

A typical aquarium RO unit produces waste water at a ratio of three to four parts waste to one part product water. This means for every litre of pure RO water you produce, three to four litres go down the drain. Higher-efficiency membranes and units with auto-flush features can improve this ratio, but some waste is inherent to the process.

Ways to Use Waste Water

Waste water from an aquarium RO unit (where the source is clean PUB tap water) is perfectly safe for many household uses:

• Watering plants (the slightly concentrated minerals are beneficial)
• Mopping floors
• Flushing toilets (collect in a bucket and pour)
• Laundry (pre-fill the machine from your waste water collection)
• General cleaning

Simply redirect the waste hose into a bucket or large container rather than straight down the drain. In a space-conscious Singapore HDB flat, a 20-litre bucket under the kitchen sink can collect waste water during an RO production run, and the water can then be used for household purposes.

This approach reduces the environmental and financial impact of running an RO system. Singapore’s PUB water tariffs include a water conservation tax and waterborne fee, so minimising waste is both environmentally responsible and cost-effective.

Storage and Handling

Store RO water in food-grade HDPE containers with secure lids. Label the containers clearly to distinguish between pure RO water and remineralised water. In Singapore’s warm conditions, stored water can develop bacterial growth if left for extended periods, so follow these guidelines:

• Use the stored water within one to two weeks of production
• Keep containers sealed and out of direct sunlight
• Rinse containers periodically with hot water to prevent biofilm buildup
• Never store RO water in containers that previously held chemicals or detergents
• Test TDS before use; if it has changed significantly, the container may be leaching minerals or the water has been contaminated

Many Singapore hobbyists produce RO water the day before their scheduled water change, storing it overnight in a covered bucket with an aquarium heater to bring it to tank temperature. This just-in-time approach minimises storage concerns.

Mixing RO Water With Tap Water

For hobbyists who want to lower their tap water’s TDS and hardness without going to pure RO, mixing RO water with dechlorinated tap water is a practical middle-ground approach.

How to Calculate Mixing Ratios

The formula is straightforward. If your tap water TDS is 100 ppm and you want a final TDS of 50 ppm, mix equal parts RO water (0 ppm) and tap water (100 ppm). The resulting blend will have a TDS of approximately 50 ppm.

More precisely:

Target TDS = (Tap TDS x Tap Volume + RO TDS x RO Volume) / Total Volume

For example, to achieve 60 ppm TDS from tap water at 100 ppm:

• Use 60 percent tap water and 40 percent RO water
• (100 x 0.6 + 0 x 0.4) / 1 = 60 ppm

This mixing approach works well for Neocaridina shrimp keepers and planted tank hobbyists who want slightly softer water than straight tap provides, without the complexity of full remineralisation. Remember to dechlorinate the tap water portion before mixing.

Frequently Asked Questions

Is RO water safe to add directly to my tank?

No. Pure RO water with zero minerals should never be added directly to a tank containing fish or shrimp. The lack of dissolved minerals creates an osmotic imbalance that stresses and can kill livestock. Always remineralise RO water to appropriate parameters before use. The exception is top-ups to replace evaporated water, where pure RO is correct because evaporation concentrates existing minerals.

How long does it take to produce RO water?

A standard 50-GPD (gallons per day) aquarium RO unit produces approximately eight litres per hour at typical Singapore water pressure. Larger units (75 or 100 GPD) produce more. Plan your water changes accordingly, as producing 40 litres for a large water change takes several hours. Many hobbyists run their RO units overnight or while at work.

Can I drink the waste water from my RO unit?

Yes. The waste water from an RO unit connected to PUB tap water is essentially concentrated tap water, still perfectly safe to drink. It has a higher mineral content than the input water but contains nothing harmful. You can safely use it for drinking, cooking, or any household purpose.

Do I need an RO system if I only keep Neocaridina shrimp?

Generally, no. Neocaridina shrimp are hardy and adapt well to Singapore tap water parameters (GH 1 to 4, TDS 50 to 120) after dechlorination. Many successful Neocaridina breeders in Singapore use straight tap water with no issues. An RO system becomes valuable primarily for Caridina species, which have much narrower parameter tolerances and require GH-only remineralisation with zero KH.

Decide Whether RO Is Right for You

An RO system is a worthwhile investment for specific applications, particularly Caridina shrimp breeding and discus keeping, but it is not a universal necessity. For most Singapore hobbyists keeping common tropical fish or Neocaridina shrimp, well-dechlorinated PUB tap water is entirely adequate. If you are unsure whether your setup warrants an RO system, visit Gensou at 5 Everton Park, Singapore to discuss your specific needs with our team. With over 20 years of experience helping hobbyists across Singapore, we can advise you honestly on whether RO is worth it for your situation and help you set up a system if it is. Read our guide to remineralising RO water to understand the full process before committing.

Related Reading

• How to Remineralise RO Water for Your Aquarium
• Aquarium Cloudy After Water Change: Why and How to Fix It
• How Often to Change Aquarium Water: Finding Your Schedule
• Water Change Temperature Matching: Why It Matters
• Aquarium Water Conditioners Explained: Prime, Safe and Beyond