ro units and ph

[rwel4809]

Hi all,

I have just set up my RO unit that comes with a TDS meter and bypass valve that allows me to mix filtered water with RO water to get a good GH (50ppm), here are the specs:

3 stage reverse osmosis wall mount unit with an adjustable membrane by-pass needle valve. This unit allows you to adjust the output hardness to the environmental requirements of your fish, by-pass water is carbon filtered for fish health, with 400 litres (plus by-pass water) per day membrane @ 80psi (output 450 lp/d at 100psi). This unit comes complete ready to start work. Membrane is a Filmtec PA (TFC) membrane Filters are 1 micron pre-filter and Matrikx CTO/2 or 10/2 carbon cartridge. Included is a dual in-line TDS meter. A proper Laundry Tee fitting is supplied standard. 5 year warranty This is a quality unit made from American components. (I also opted for a special Chloramine cartridge as I'm in Sydney)

I have found some odd results..

My untreated (Sydney) tap water has a PH of 7

The pure RO water has a PH of 5.5 ish

The mix of RO and filtered water has a PH of >7.8????

I thought that the mix would be lower than the tap water???

I am hoping the PH will come down in my (soon to be established) planted tank with the aid of C02... Do you guys think I should try to buffer the water down before adding it to the tank??

I am trying to stay as chemical free as possible!!

Thanks..

Robert

[ILLUSN]

sounds like some kind of tempory harness leaching from the column, should dissapear after a few runs.

[fishgeek]

measuring pH in pure water is eceedingly hard to do accurately
because there should be little to no reason for dissociation into H+ ion or OH- ions

alkaline water has more hydroxly ions

what i imagine is happening is you have a tendency to alkaline wtaer from teh tap and a lt of stabilising products aswell

when removing most of this in the r/o unit you allow the water to be manipulated much more easily
ie a couple of drops of acid send the pH dropping rapidly
equally a bit of your tap water see's the pH rise rapidly

dont be tempted to be too worried by this, once in your tank the r/o has less buffer(you know that, and that is why you are adding tap back to it) in the tank the denitrification process will tend to acidify, low budffered water will swing that way more easily

as long as you practice regualr water changes you wont see marked pH fluctuation in the tank
maybe if you are still unsure make up some of your change water as you were.. ie r/o and tap.. that is very alkaline and then add tank water to it and see if it ends up being more neutral

i had exceecdingly hard tap water and used to run my ro unit into a 220 litre water butt with peat in... and just use that straight in my tanks
sometime i would add a bit of straight tap back
in my situation i assumed 90-95% efficency of my unit
started with gH of about 20 and kH of about 14
assumed after filtering gH about 1.5 kH about 0.7

my pH stayed stable and again i assumed that even with such low measureable buffer the natural products released from the peat were stabilising the water pH at 5.5-6.0
i often had peat in the tanks too

basically understand that r/o water is difficult to measure pH in accurately
and that it is much easier to manipulate with other products

hope that helps rather than confuse's

[fishgeek]

sorry i should have read more carefully

why do you even want pure water in aplanted tank?
i assumed you were having discus

plants use the elements you are removing from the water with the filter
CO2 will have marked effects on pH in r/o water

try it with your unit, a soda syphon or just some plain old soda water
dissolution outgassing etc will make pH fluctuate

have i got the wrong end of the stick again?

[rwel4809]

why do you even want pure water in aplanted tank?
i assumed you were having discus

Thanks for your advice!! much appreciated!!

I do plan to have a planted tank - for discus...

The reason I went with the RO unit is the very bad quality of Sydney tap water. As soon as it rains heavily, when the dams are low, there is a bacterial bloom and Sydney Water ups the chloramine to compensate. They put so much in that dechlorinator cannot remove it all... I found this out to my disgust when I kept loosing fish after water changes.

The unit I have filters all of the chloramine out and the RO bit reduces the poisonous heavy metals..

If the PH is likely to be unstable in a tank with C02 added, maybe I should put some shell grit in the filter?? what do you think??

[tzwms]

It isn't that the CO2 will make the pH unstable, but more that the CO2 will have a big effect on the pH. You need to add something to raise the KH (buffering capacity) which will be near zero in the RO water. You can use shell grit which will also raise the GH as well or sodium bicarbonate (baking soda) which will raise the KH only.

HTH

:)

[fishgeek]

if you are just concerned by the heavy metals and chloramines then you should have just got the filter without the ro membrane... you can still bypass it

that way you will remove your chloramine and heavy metals and not waste so much water at the r/o stage

your water will still have a similar hardness as tap and should be healthier for your fish


andrew

[ozarowana]

I agree... look at the caravan system from the same supplier. You don't need RO for what you are trying to achieve.