# [Wet Thumb Forum]-RODI water, pH and water hardness



## Mark Spencer (Apr 19, 2004)

In my quest for perfect water for my planted 50 gallon aquarium I have started using RODI water (reverse osmosis, deionized). Since I already a RODI filter for my reef tank it cost me nothing to switch to it. Of course, this water has zero hardness so I added baking soda (one teaspoon per 5 galons). When I do my pH rises from 6.8 o 7.5 and the dKH increases from 0 to 4. I also inject CO2 using a yeast container.
Are these pH, dKH conditions acceptable to fish such as cardinal tetras which are native to soft, acidic water? (I have killed every cardinal and neon tetra I ever puchased.) Is there any benefit to using RODI water?


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## Mark Spencer (Apr 19, 2004)

In my quest for perfect water for my planted 50 gallon aquarium I have started using RODI water (reverse osmosis, deionized). Since I already a RODI filter for my reef tank it cost me nothing to switch to it. Of course, this water has zero hardness so I added baking soda (one teaspoon per 5 galons). When I do my pH rises from 6.8 o 7.5 and the dKH increases from 0 to 4. I also inject CO2 using a yeast container.
Are these pH, dKH conditions acceptable to fish such as cardinal tetras which are native to soft, acidic water? (I have killed every cardinal and neon tetra I ever puchased.) Is there any benefit to using RODI water?


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## nino (Oct 2, 2004)

I use RO water for my discus tank. What is your tap water parameters (pH, GH and KH? Sometimes it's simpler just to mix RO with tap to get some minerals and KH from tap. RO/DI strips all minerals from the water. I mix 75% RO with 25%tap. As you may know already, using baking soda will only raise the KH and not GH.

My tap has pH of 7.8, GH over 300ppm and KH of 6.7. In my case, I need some RO water to keep some sensitive fish like discus. But I don't use RO water in my planted tanks with a lot of neons in them. I inject CO2 and my tanks'ph are around 6.9 but still very hard (high GH). Neons and some other tetras are famous of being weak. This is caused by massive inbreeding by hatcheries. So it's not always your mistake that they died. All my neons are still in hard water and they are doing fine. Try buying from other stores


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## Roger Miller (Jun 19, 2004)

The best water you have for your planted tank is probably the water that comes out of your tap. The minerals in water are essential plant nutrients and they are usually present in natural proportions that are essential for good plant growth.

Straight RO/DI water is poison to plants. It is possible to reconstitute the normal composition of water from RO/DI. That's something you might do if your water supply has a very unfavorable electrolyte balance. If your water is just hard then use your RO/DI to cut the tap water.

Cardinal tetras can live just fine in water that isn't soft and acidic. They just won't breed.


Roger Miller


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## Mark Spencer (Apr 19, 2004)

My decision to use RO/DI water was based, perhaps erroneously, on my experience with my reef tank. The phosphates in tap water invariably create an algae bloom in such tanks. I have fought algae in my planted tank for some time so i tried RO/DI water to combat it. I was also concerned about chlorine and the use of chelating chemicals (Aquasafe) to combat it.
My pH is 7.5 and dKH is 4. My tap water is very soft (dKH 0) so, even if I use it for water, I need to add to the hardness. In such a case is baking soda the best course?
I don't currently measure general hardness. Should I do so and what value should I aim for in a tetra/planted tank?
From the two responses I've gotten I'm starting to understand that the use of RO/DI water is unwarranted.
Thanks for your responses,
Mark Spencer


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## nativeplanter (Jan 27, 2004)

Mark,

I use DI water in my tank for the same reason you are inclined to - high phosphorus at tap. Get some Kent R/O right, or a similar product, to reconstitute the necessary ion concentration for your fish and plant. Roger is quite right that using straight DI water will kill both fish and plants unless it is reconstituted. However, once you reconstitute it, you will not have to add ions again unless doing a water change. Make-up water (for evaporation) can be straight DI water.

-Laura


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## Roger Miller (Jun 19, 2004)

> quote:
> 
> Originally posted by Mark Spencer:
> My decision to use RO/DI water was based, perhaps erroneously, on my experience with my reef tank. The phosphates in tap water invariably create an algae bloom in such tanks. I have fought algae in my planted tank for some time so i tried RO/DI water to combat it.


Transfering your background from reefs to planted tanks may prove to be a problem. Probably the greatest crossover is from a reef tank to a low-tech planted tank.



> quote:
> 
> I was also concerned about chlorine and the use of chelating chemicals (Aquasafe) to combat it.


 Concerned in what way? Various dechlorinators have been used for a long time without widespread problems. Many (most?) made-for-the-hobby plant fertilizers contain chelating agents. Some substrate materials include and release agents that create stable complexes with lots of things.



> quote:
> 
> My pH is 7.5 and dKH is 4. My tap water is very soft (dKH 0) so, even if I use it for water, I need to add to the hardness. In such a case is baking soda the best course?


No. If your water is that soft then you need to add calcium and magnesium along with the KH. You would do that with dolomite or with a combination of calcium carbonate and magnesium sulfate.



> quote:
> 
> I don't currently measure general hardness. Should I do so and what value should I aim for in a tetra/planted tank?


You do need to monitor general hardness in a planted tank -- particularly if your water is soft. Four degrees GH is a good target. More is probably better. I think all tetras can be kept with 4 dGH. Many tetras will even breed in planted tanks with that hardness. If you want to breed tetras that really need softer water to breed than they are probably better kept without plants.



> quote:
> 
> From the two responses I've gotten I'm starting to understand that the use of RO/DI water is unwarranted.


Possible, but not warranted.

Roger Miller


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## Mark Spencer (Apr 19, 2004)

Thanks again for all your helpful responses. This is what I conclude from the feedback:
1. A planted tank is not a reef tank.
2. Tap water contains minerals (esp. Ca and Mg) that are valuable for planted tanks.
3. The only reason to resort to purified water, such as RO/DI water, is if there is an unusual electrolyte balance or if the phosphate concentration is so high that it is creating an algae problem.
4. If one does use RO/DI water then one must add minerals via a product like Kent R/O right or additions of CaCO3 and MgSO4.
5. Carbonate hardness is needed for moderating pH swings but general hardness is just as important for maintaining mineral concentrations (Mg, Ca and K).
6. I will ditch the RO/DI water for now unless and until I experience an algae bloom.

Is this correct or am I stil confused?
Thanks again,
Mark Spencer


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## Roger Miller (Jun 19, 2004)

Mark, you got that down pretty well.

The only item I would modify is #6. If for some reason you do have an algae bloom then resorting to RO/DI is probably not the answer.

Most tanks have at least a small algae problem during their setup period. The algae population stabilizes once the tank and your management practices settle down to a stable routine. The "best answer" in this case is to wait. When you have a stable algae population then you can tune your routine and your algae-eating population to match.

In any planted tank (new or old) the secret to long-term algae control seems to be to keep a dense population of healthy, growing plants. For most purposes the "best answer" is to get your plants in that healthy, growing state and keep them there. More than anything else that takes good observation and a few gardening skills.


Roger Miller


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