Reverse Osmosis

[jowens]

Hi Guys,

I am receiving a RO unit in the mail any day now. I'd like a little advice on getting the kH right for the fish I have if possible. The two relevant species in my tank are A. Agassizzi and A. Cacatuoides.

A few facts:

- My tap water is exceptionally hard. I don't have an exact measurement, but will get one when my sensor comes with the RO unit.

- I use CO2 injection on my tank, which makes the water more acidic then it ordinarily would be for the kH it's at. It will also make me more prone to pH dives once I start using RO water.

- I make weekly 20-30% water changes.

- It is a newly planted tank that appears to be doing well, but I don't think the plants are creating optimal oxygen levels yet.

- There is a relatively light bioload (about 15 fish - most under 2.0 inches - in 46 gallons).

- All fish in the tank prefer soft/acidic conditions, though I don't know that many would like going much lower than 6.3 pH.

I have two questions:

1. What kH do you guys think I should aim for when I start using RO water for changes...to keep the Apistos happy while still avoiding the pH crashes associated with CO2 injection? I've been told 3 kH is probably as soft as you want to get with CO2 injection. Note that the water will be more acidic then ordinary for any kH I arrive at.

2. What method do you favor for buffering RO water? I've been told that adding a buffering agent to 100% RO water (that has been aerated) is better than mixing RO water with tap, since it's more precise and avoids any additives that might be in tap water.

Thanks,
Jason

 

[jowens]

I seem to have answered my first question: most people agree that 3kH is a good number to shoot for, since the CO2 will make my water a bit acidic anyway.

I'm still curious about chemical buffering vs. mixing with tap though...

 

[Neil]

Jason,
There is a triangular relationship between pH, KH, and CO2. It makes the problem of pH a more difficult one to deal with, but it also provides a benefit, which makes life easier. Most people have a tough time finding a way to get their pH down. I think that it is a valid point to shoot for 3KH with a pH of 6.3. Regardless of the level of CO2 you are adding to your tank, as long as you have some buffering element (like a KH of 3), you should not have a problem. Through water changes you are hampering most of the elements that will work together to drive your pH down further. I am no scientist and this is far from one of my better subjects, so I would love someone who knows more than I do to comment on this issue. But I do know that there are many different buffers (store purchased acid and alkaline buffers, the level of KH, peat, various types of substrate like fluoride, and a number of others). I don't know about the acid buffers available in the stores, but I have heard that people have used them to great success. A 6.5pH acid buffer would probably keep your pH between 6 and 6.5, even if you had less KH and injected CO2 fairly heavily. But again, I welcome comments from others that really know. Neil

 

[aspen]

beware using acid buffers in a planted tank. read the label: phosphoric acid= phosphates= ALGAE! try a tapwater mix, with your r/o at first. beware being too soft, esp in a planted tank. the plants eat carbonates, and when this is gone, the ph can crash. also nitric acid is created by the nitrification process, which will also consume carbonates.

what are you using for your co2 addition? are you injecting with a tank or diy. imo, a tank fed system is essential when going real low with the hardness, so the addition is stable. i use a diy system and the co2 added varies quite a bit, whether it is new, or runnning low. go higher in the kh value, then slowly adjust it down with water changes, until you are happy. monitor closely, until you are sure there is no problems. always keep a box of sodium bicarbonate (baking soda) close at hand, and watch for the telltale signs of a ph crash. ie the fish freaking out, gasping for breath.

i have tried to fool with the ph of my tanks and consider it to be a real pain, what with ph swinging all over the place. i am using straight tapwater in all of my tanks at present.

rick

 

[Neil]

jowens,
Have you gotten your RO yet? How is it working out? It is one of the best tools a Dwarf nut can have, besides patience and a sense of humor . N

 

[jowens]

The RO Unit is taking a while. It is supposedly arriving today. I'll leave it at that!

When I get it, I'll be excited to just get my water in the *somewhat acidic* range. It's so basic right now it's rediculous. It will probably be some time before I really start trying to push the kH down low enough to risk a crash. But I'll follow the advice of slow adjustments with lots of careful monitoring.

As a whole the tank is just starting to stabilize after being planted and having a new filter added on in the last six weeks or so. It's been fairly traumatic for all involved, including myself, but I've managed to only lose one fish in the process.

 

[davemill]

I reconstitute pure RO with Kent's RO Right, which provides GH. Then I maintain my tank at 2 KH. With 80-100 bubbles/minute of CO2, that gives me a pH of around 6.5 and 19ppm CO2.

I add 20 gallons per week into my 72 gallon bowfront. This means I'm removing about 16-18 gallons, plus 2-4 lost to evaporation. My reconstitution formula for 20 gallons is:

2 teaspoons RO Right
1 teaspoon Sodium Bicarbonate or baking soda or Kent pH Stable (all the same thing)

KH will drop slowly during the week, as carbonate is consumed. It starts at 2 or so, and never drops all the way to 1.5. But if I forgot to change the water for a couple of weeks, I would be setting myself up for a pH crash. I am considering raising the KH target to 2.5 to increase dissolved CO2 at the same target pH.

 

[Neil]

davemill,
Great info! A formula! 8)