Changing pH, gH, and kH
If you are sure you need to use RO water in your aquarium the first thing you must be aware of is that RO can’t be used “neat”. Fish need some minerals or trace elements to be healthy and thrive RO has virtually none. Carbonates are necessary to prevent pH swings and enable filter bacteria to function. You will have to put minerals and trace elements into the RO water to make it habitable.
Remineralising RO water by mixing with tap water
You can remineralise RO water very simply by mixing it with tap water. You can halve the gH if you mix 50:50 RO and tap water together. Lowering pH is harder because the kH value needs to be considered. kH is what buffers the pH and if very high, pH may not reduce enough. If you keep fish that need very soft acidic water in a very hard water area, your mix may contain so little tap water that you will find using all RO plus minerals to be a better option.
Using 100% RO water with added minerals
Minerals can be added to pure RO water creating ideal conditions for the fish it will be used for. There are many products available: some are general mixes that raise both gH and kh, some raise only one or the other. You can also buy mineral mixes designed for certain types of fish: mixes for Discus, Rift Lake Cichlids, and others.
Hardness is more important than pH when it comes to remineralising, and the most important factor is having the right TDS (total of dissolved solids) for the fish you want to keep. If you know the TDS you need you can use an all in one product such as TropicMarin Remineral Tropical or JBL Aquadur for example. All you need do is to mix enough of the product into your RO to reach the required TDS value using a TDS meter. The pH is less important in this method and all in one products add both gH and kH at the same time.
Where fish need either very low or very high alkalinity, separate products for gH and kH might be a better choice. Again there are many options including mixing your remineraliser from commonly available materials. Add remineralisers to your container of RO the day before a water change, with a heater and a powerhead to heat and aerate the water before use.
Vital Points to consider when switching to RO Water in your aquarium
When you first switch to using RO you MUST do this gradually over a number of partial water changes. Changing hardness and/or pH too quickly is very harmful and can be fatal to some sensitive fish.
Once the change has been made you will need to continue with your new RO mix at EVERY water change. Suddenly going back to straight tap water is not an option. You will need to ensure you always have a supply of RO water and any minerals and adjusters to hand.
Your RO water mix must be consistent. Experiment and make notes until you can achieve the same parameters every time. Until you can be sure of being consistent, don’t start adding your mix to the tank
Using RO can be costly, time-consuming and requires a commitment to accuracy and to continued use. You can buy it from your local fish shop (LFS) or buy an RO unit to make your own. Both have drawbacks and benefits. If you only need small quantities for one tank, buying from an LFS might be the way to go. For larger quantities, installing your own RO unit will work out cheaper.
If you choose to make your own RO, as well as the unit itself you will need a TDS meter, suitable food-safe storage containers, a heater and powerhead for mixing minerals and raising the water temperature and a plumber to fit the unit unless you are confident with DIY plumbing. The prefilters and RO membrane will need to be replaced regularly.
The amount of water sent to waste can be high – typically for every litre of RO produced, four litres goes to waste consequently on a metered supply this can add a fair bit to your water bill. You may need to inform your water supplier of your intention to install an RO unit and additional charges may be made for water drainage. You can be required to have a meter installed if you do not already have one.
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More on water chemistry and quality:
- RO Water for Aquariums
- Fishless Cycling Guide
- What is Fishless Cycling?
- What is Reverse Osmosis (RO) Water? Why Would I Use RO?
- Dealing with Ammonia and Nitrite Spikes in Aquariums
- Dealing with high nitrates in aquariums
- Is nitrate control really necessary?
- The Nitrogen Cycle in Aquariums