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Using reverse osmosis (RO) water for your angelfish. What you need to know.

Updated: Apr 10, 2019

With the availability of RO units in just about every hardware, aquarium and water specialty store, it's never been easier to have full control over the water that you put into your aquarium. Reverse osmosis units install in minutes, and let you control your pH and hardness more accurately, but they come with cautions that you need to be aware of for the health of your fish.

99.99% pure water. Do you really need it?

I ran a aquarium store with over 700 aquariums for a few years where we sold a lot of reverse osmosis water, but never used it once. For our purpose, we didn't need it. RO water is great if you need to top up evaporated water in sensitive aquariums (reef tanks), if you are striving to raise delicate fish like discus or altum angels to their full potential, or if your tap water conditions are toxic to anything living (unlikely). The average aquarist does not need it, and when it is used improperly it can have some destructive results. There's a handful of pointers we'll discuss, but to understand RO water, we need to know what water with 0 GH and 0 KH means for your aquarium and how it impacts pH and your fish. We'll also discuss TDS. Don't worry, we're tackling this from a high level.

What you need to know

GH is how hard or soft your water is

#Generalhardness (GH) is a measurement of the dissolved magnesium and calcium ions in your water. It's measured in German degrees, or °dGH. In order to get the measurement to ppm, multiply the number of degree by 17.9 and you've got it. Discus, altum angelfish and other extreme soft water fish like to be in the 0°-3° range. For your standard community aquarium, 3°-11° is an acceptable range to keep almost all fish happy. African cichlids and hardwater fish like their range to be 11°-22°+

#KH is your water's ability to buffer pH

This is a measurement of the dissolved carbonate and bicarbonate ions in your water. This does not affect your general hardness. It's also measured in German degrees, with the same conversion to ppm as GH.

When you're adjusting the pH of your water, you're fighting with your KH. The KH is working to keep your water's pH buffered, and it's absorbing the acid you're adding. Eventually, you'll overload the KH, like soaking a sponge, and your pH will swing easily. If you have a high KH level and you're trying to lower the pH, you're going to be spending a lot of money on products to help you achieve this.

TDS is an overall measurement of "stuff"

Yep, it's not that useful for aquarists. #TDS stands for "Total Dissolved Solids," it includes organic and inorganic matter and is measured in ppm. Calcium, magnesium, nitrates...everything, but the problem is that you can't see which value is affecting it most without further testing. For example, taking a TDS reading is the equivalent of someone telling you a room is cool and asking how to warm it up. You don't now the temperature in the room, you don't know the reason for the cold air, and you don't know how the person asking you is dressed. A TDS reading of 200 can have 50ppm of ammonia, or 100ppm of nitrates, or neither. You can't tell without looking into it further.

Now, you can do a nifty trick with TDS, you can get a generalized baseline of your aquarium's health based on past observations. For example, I go test my TDS in my aquarium and see that it's showing 110ppm. I know that my aquarium has a functioning biofilter, so ammonia and nitrite are broken down to nitrate efficiently and should not be adding any ppm to my reading. I take a nitrate reading and see that I have 10ppm, this means, and I'm making some big assumptions, that if my TDS reading goes up to 140ppm in a week, I likely have added 30ppm nitrates to my aquarium for a total of 40ppm nitrates. Really, take TDS readings in your aquarium with a grain of salt.

TDS meters are a fantastic tool for checking the health of reverse osmosis filter, which at peak efficiency should read 0ppm. As you use the RO filter, it becomes less efficient and the TDS levels start to increase. TDS testers are cheap, easy to use, and provide instant readings.

How to use RO water in your aquarium

Manipulating the pH of your aquarium

Because RO water has no KH, it's really easy to change your aquarium's #pH without using a too much product. That being said, you must buffer RO water or you risk suffering a pH crash and killing everything in your aquarium. #ROwater has a pH value between 5-7, and it swings. There's nothing in the water to keep it stable. Luckily, there are some great products out that make life easier. I use Discus Buffer and Neutral Regulator by #Seachem to lower and raise the pH of my water. There's a ton of other buffering products they provide which will get both your tap water and RO water to the levels where you want them.

Before getting a reverse osmosis unit, I needed to use 1/4 of a liter of #discusbuffer on a 55 gallon drum of water because my high KH level was fighting the adjustment. When modifying the same volume of RO water, I use about 3-4 tea spoons. That's a big difference in product, saving me money over the long run. Doing some quick math, that's about $30CAD a month x 12 for $360 in product. A RO filter can cost as little as $100 and come in different styles for to suit your installation needs.

Softening your aquarium's water

Reverse osmosis water, from a fresh filter unit, should measure 0 KH, 0 GH and 0 TDS. I like to think of it as white paint. It has no value except for its substance and it's a blank slate to work with. If you're aiming to create an environment with #softwater in your aquarium, there is no more effective and controllable way than RO water. Water softening pillows typically don't work well (and create other issues) and aquarium peat is unreliable in its application, and provides results that are difficult to replicate consistently. Yes, #peat has it's benefits, but there's no measurement to know how much to use to make two tubs of water match each other's parameters, and how long it will take to get the results you want. RO water lets you start sciencing right away, accurately.

Replacing evaporated water

Your aquarium's water is constantly #evaporating, and RO water is perfect for topping up your tank. If you have a lid on your tank that fits well, you probably see very little water loss. Open top aquariums can lose a significant amount of water every week and when they are topped up with tap water, it starts to bugger up the water parameters. Evaporated water leaves behind all of the minerals and nutrients it was diluting, so by adding more tap water full of minerals you are slowly increasing the hardness of your aquarium water over time. Reverse osmosis water replaces only what was lost...clean water.

Things to consider before using RO water

More "stages" doesn't mean better

When shopping for a reverse osmosis filter, there are a lot of options and brands on the market. Historically, RO filters were 3 stage - a sediment filter, carbon filter, and RO membrane. Then someone got the idea for a 4 stage, then 6. If you google "9 stage RO filter," weird monstrosities come up. A 3 or #4stage filter is all you require for your aquarium, with the 4th stage being a D.I. cartridge. Ensure that the filter you purchase does not have an alkaline or remineralization cartridge. This defeats what you're trying to accomplish and is geared toward drinking water.

pH swings can kill your fish

#pHvalues are based on a logarithmic scale, which means that an aquarium with a pH of 6 is 10x (or 1000%) more acidic than an aquarium with a pH of 7. That's a very large shift in acidity that your fish do feel, and any #pHshift more than a 0.2-0.3 in a 24hr period begins to stress your fish. Ensure that the water you're adding to your aquarium matches, or only slightly manipulates, the pH. For example, if I have an aquarium with a pH of 7.5, and I want to bring it down to 6, I would do a 20% water change on the tank with water that has a buffered pH of 7.1, aiming to bring my aquarium down to 7.3. Next time I'd do another 20% change with water that has a pH of 6.9, hoping to bring my aquarium down to 7.1. I'd repeat this process until I hit my desired levels.

Use a buffering product that brings your KH to around 4 degrees. pH crash happens when there isn't enough carbonate hardness to buffer, and acidic elements (eg. C02, fish waste) inside of your aquarium can force the pH to drop rapidly and dangerously.

Water changes need to be planned for

Reverse osmosis units typically produce 75-100 gallons per day, dependent on the unit. To make sure you have enough water to change your aquarium, you'll probably need to run your unit for 12-24 hours. Get a cheap, food grade 55 gallon #drum ($20), or a strong container from Home Depot ($10) to collect your RO water, heat it to the temperature you need, and buffer it outside of your aquarium. I tend to keep a 55 gallon drum full of RO water in case of emergency for my 180 gallon altum breeder aquarium. If something were to happen, at least I know I can do a ~30% water change immediately.

Not for the average aquarist

Reverse osmosis filters are a great tool to help advanced aquarists manipulate their fish tank’s water. If you’re doing a specialized breeding project with fish that require soft, acidic water, there’s no better tool to help you on your journey. As for regular aquarists who want to enjoy their fish tank and not concern themselves with difficult fish that have specialized requirements, don’t play with RO water as it can be just as dangerous as it is helpful.

Leave a comment, a question, or even a contradiction down below and we can learn together.

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Andrei Vexler is an aquarist with over 20 years in the fish hobby. Having run a fish room with over 700 freshwater and saltwater aquariums, Andrei found his passion in South American cichlids, particularly altum angelfish. Growing and wholesaling angels to the GTA and surrounding area, he shares his years of experience in his blog for advanced aquarists.