Is PH harder to maintain in RO water?

[dereks]

This tank is cycled and I have 3 corey cats in a 20 gallon (long), I have 2 pieces of malaysian driftwood and good filtration.

I have been using RO water in this tank. I checked the PH a week ago and it was 7.0, a week later before my water change it was 7.4. After the water change it dropped back down to 7.0.

Does anybody know what could cause a swing like this? In my experience my aquariums don't swing so drastically, could this have something to do with the RO water? That is the only thing I'm doing differently in this tank. Is it possible she's feeding them too much?

Do nitrates have an effect on PH? Do plant ferts? (seachems)

Thanks!

 

[esoepr1976]

Are you adding anything to the RO water to put back the needed minerals for the fish? Why are you using RO water in this tank? Anything in the tank besides the cory cats? What is the ammonia, nitrites and nitrates? What would the PH of the water be if you weren't using RO water?

Emily

 

[dereks]

The only thing I'm adding to the RO water is the fertilizer. I'm using RO water to try to keep the PH low for the cardinal tetras. There is nothing in the tank besides the corey cats. No ammonia or nitrates, haven't checked the nitrites.

Do you mean what is the PH of my tap? 7.4

Do nitrates have an effect on PH? Do plant ferts?

 

[digitalphrage]

Lots of things alter the pH of your water (you probably already know that). If you're using RO water, it's better to add back in some tap water or use some of the pH buffering chems to keep things stable. There are also non-chemical additives you can use to adjust pH up (oyster shell, aragonite) or adjust down (driftwood, peat).

I would think that ferts could cause pH to change -- some minerals like calcium and magnesium are going to buffer the pH to higher levels and I *think* iron will lower the pH. Depending on the composition of the fertilizer, it could potentially go either way. Even fish foods and decomposing waste can alter the pH. I'm not sure of the pH effect from nitrates.

If you've got a heavily planted tank, you will even notice a pH change between day and night due to the fact that plants use carbon dioxide during the day, but not at night. As CO2 depletes, the pH will rise.

The more heavily buffered your pH is, the more stable it will be.

That being said, a pH shift from 7.4 to 7.0 probably isn't going to be an issue for anything in the tank, though others might have more experience or knowledge about that and disagree.

If I had to guess, the pH shift you saw may simply be due to the CO2 and demineralization factors. It's possible that there was more CO2 in your RO water than in your tank, and probably more buffering minerals in your tank than in the RO.

 

[TheFishBoss97]

+1^

also, what is the pH of the RO water straight out of the tap...could have something to do with filter in RO system(not likely, but possible)...

 

[OrionGirl]

You need to know the GH of your water. Straight RO water won't contain enough buffer to maintain a stable pH, but knowing the GH will help you determine what needs to be done to keep it stable. Stable at 7.4 will be better than fluctuating levels--and the cardinals will be fine at that level.

 

[dereks]

digitalphrage,

Ok so basically what I'm hearing is I need to add back in "PH up" to level it back out? If that's the case I'll just switch back to the tap water because that would defeat the purpose of me using the RO water. The idea was to keep the PH down for the cardinal tetras I'll be adding.

And I guess I'm still confused on the bottom line here, is RO water more difficult to keep a stable PH than most North American tap waters?

Also, I guess I assumed that a .4 swing was a pretty drastic swing, is this not the case?

 

[OrionGirl]

Yes, RO water is more difficult to keep at stable. It lacks the buffers present in most water supplies.

.4 is a huge swing. The reason it may or may not be a big deal is that fish don't read pH. They react to the TDS changes that may or may not accompany a change in pH. It's important to understand WHY pH is changing. In any case, stable is better than specific unless you're trying to breed the fish.

 

[digitalphrage]

digitalphrage,

Ok so basically what I'm hearing is I need to add back in "PH up" to level it back out? If that's the case I'll just switch back to the tap water because that would defeat the purpose of me using the RO water. The idea was to keep the PH down for the cardinal tetras I'll be adding.

And I guess I'm still confused on the bottom line here, is RO water more difficult to keep a stable PH than most North American tap waters?

Also, I guess I assumed that a .4 swing was a pretty drastic swing, is this not the case?

No, the PH Up and PH Down products are really not great ideas to use, I believe they cause fluctuating pH and don't keep things stable. If you're going the chemical route, look for pH buffers. Seachem makes a variety of such products (there are probably other manufacturers as well) -- acid buffer, neutral regulator and several products for buffering the pH at higher levels. The closest analogy I can think of is cold medication -- those ph up/down are like medicines that cover up the symptoms of your cold, where as the buffering products are like the antibiotics that actually treat the underlying cause of the problem.

Adding pH buffers to your RO (whether in the form of the buffering chemicals, driftwood, leaves, tap water, etc) doesn't really defeat the purpose of RO. Depending on your tap water quality, you're still going to come out way ahead in terms of reducing the amount of dissolved solids and pollutants if you stick with RO -- even if you're adding back in a proportion of tap water.

Yes, RO water is more difficult to keep stable -- there's nothing in it to buffer (stabilize) pH because virtually all of the buffering components that typically stabilize pH have been stripped out.

 

[digitalphrage]

I forgot to add this. I have a couple of tanks that I keep in the 6.6 - 6.8 pH range. When I do partial water changes, I exchange out about 2 - 3 gallons at a time. The water I use to replenish is about 1 part tap, 4 parts RO or rain water. The pre-treatment pH is usually around 7.8 - 8.2. Since I mix the water up about 10 gallons at a time, I add around 1/8 to 1/4 tsp of the Seachem acid buffer and test the pH of the water going in. It's usually between 6.0 and 6.4 (solution turns slightly green-yellow).

For me though, I don't make this an exact science, sometimes I use a little more tap, sometimes the water I mix up ends up in the 6.6 to 7.0 pH range -- because I'm only doing partial water changes, it doesn't ever drastically affect the pH in the tanks.

Also, this is a deviation from the directions for the acid buffer. The packaging recommends using acid buffer in conjunction with their alkaline buffer products. If I were using straight RO, I'd use both products, but since I mix in varying amounts of tap, I've never found it necessary to include the alkaline buffering product to keep pH stable.