Should I use RO/DI?

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Miller2112

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Nov 2, 2020
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Hello all,

I am new here and new to freshwater. I have been keeping reefs for many years. I am planning a 29gallon planted tank. I have been looking around to see if it was worth using my RO/DI unit for freshwater or is it to "clean" for freshwater? I know with reefs im adding all the stuff i need with my salt mix not sure with freshwater though.
 
Apr 2, 2002
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Usually in FW RO/DI is used mostly to fix bad tap water. Then there are some fw fish where you need it to recreate their needed parameters. Some people may use it as part of a rainy season to induce spawning in certain n species. I use it for Altum angels.
 

Miller2112

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Nov 2, 2020
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Thanks for the info. I haven't tested my tap water in years so i guess i will. If i use RO/DI i assume i have to add trace elements back in?
 
Apr 2, 2002
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No, you cannot conclude that until you know what is in your tap and then what fish you want to keep. RO/DI can be used in multiple ways. Many people mix it and their tap to produce a desired result.
 

Miller2112

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I tested the PH of my tap water and it was 7.4. I tested the PH of my RO/DI water and of the tank and it was 7.8. Not sure what to do with that since ive read that testing PH in RO/DI water is not accurate since everything has been stripped out of it. This is definelty not an aspect im used to. With my reeftanks my PH is 8-8.2 with no effort at all from me.
 
Apr 2, 2002
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Actually, what happens with RO/DI is that it has 0 KH. KH is what hods up pH. So when one makes RO/DI it should be dead neutral. But it is not, it actually turns acid pretty fast. This is due to the carbon dioxide which the water naturally absorbs from the air. This in turn creates some acid in the water. Since there is no KH, the pH of the water drops.

I use a digital testing for my one RO/DI added tank. I mix 20 gal. of changing water which is 11 RO/DI and 0 tap (7.0 pH and 83 ppm TDS). The testers really hate pure water in terms of the TDS (conductivity). I never put the probes into the changing water until I have got it all mixed save for any need to lower the pH with muriatic acid. This may be done based on where the tank parameters are when it its time to do the water change. Between water changes my water moves up from the 5.8-6 pH I start it at towards 6.5.

The most important consideration has to do with the bacteria and the cycle. Despite what one often reads on fish sites, it is not true that nitrification stops if the pH falls under 7.0. What happens in such cases is that the bacteria have to change their source of energy from ammonia (NH3) to ammonium (NH4). As the pH drops, the ammonia (NH3) levels head towards 0 and all the ammonia is in the form of NH4. The bacteria mostly have receptors for ammonia, but they also have some for ammonium. So they are able to switch over. However, they do not process ammonium with the same efficiency. This process takes time and, in the normal cycle, it will appear that the ammonia have stopped working entirely.

However, a bit of patience and one sees that the cycle may be interrupted, but that it then resumes. Since most fish keepers are not working with water much under a neutral pH, what they see is a stalled cycled. The one side benefit to all of this is that ammonium is a whole lot less toxic than ammonia.
 

Miller2112

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Ok that all makes sense to me. I just need to lower the PH on the tank since its at 7.8 and everything im reading says thats to high.
 
Apr 2, 2002
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Too High for what?

African cichlids live in water with a higher pH then that. Usually, the TDS are more important than the pH.
Many live bearers work in your pH. Some corys will as well.
Also, parameters given for fish are usually for that fish in the wild. Tank raised (or farmed) and even some wild fish can do fine a bit outside of their normal parameters. The biggest risk is that they may not spawn successfuly.

Here is one thing you can try to get a handle on your tap water and potential mixes with RO/DI. Buy a gallon of distilled water. Since you have your tap parameters, you can make up a few test mixes. Mix 4 ounces of tap with 4 ounces of distileed. Shake it up or bubble it with an airstone to get the gasses in balance. Then test to see where the pH, GH and KH are. You can do this with several different mixes. This will give you an idea of what mix it will take to get near what you want.

It is a lot easier to raise the pH and TDS and hold them there than it is to do things in the other direction.
 

Miller2112

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Nov 2, 2020
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I am just planning a basic community tank. Gourami maybe some rumynose they look cool, Havent thought to far into stocking but nothing crazy. Not worried about spawning or anything.
Everything i was reading made me believe 7.8 was to high.

Rather then buy distilled can i jsut do mixes of my RO/DI and my tap to try to get someting more inline with what i need?
 
Apr 2, 2002
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Sorry, yes use your ro/di. I am work with a few folks on stuff and it slipped my mind you had the ro/di unit even though you posted that.

You can do a bigger mic than 8 onces since you have the unit. It's a lot easier using a half or a full gallon and likely more accurate.
 
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