Water Parameters - Sanity Check

[Badger]

Just wanted to run these figures by some sane people. Check my math and/or knowledge of parameters.

History: I have well water, put through some sort of spendy filtration system at the source (tap water) and I have an RO unit. I'm trying to get my freshwater tanks going again and I'd like some plants, so I'm testing and trying to get this science down. Here's what I found so far. (I'll test again in a couple days after the water has sat for a while.) I used new API drop tests for all of it, and calculated CO2 with http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm and "Chuck's Planted Aquarium Calculator".

Tap Water (fresh from tap)
pH: 6.5
KH: 14 drops (degrees)
Calculated CO2: 132ppm

RO Water (fresh from RO unit)
pH: < 6.0
KH: 3 drops (degrees)
Calculated CO2: 113ppm (Assuming that pH is 5.9)

Tank Water (Sat for a few days in my aquarium, no fish, just a filter and a big piece of driftwood for decoration)
pH: 7.6
KH: 12 drops (degrees)
GH: 3 drops
Calculated CO2: 9ppm

Does any of this look wrong to an expert? Never really been into testing KH before, so I'm a noob. I appreciate any help. Thanks!

A day later...

RO Water (been sitting for about 24 hours)
pH: 6.2
KH: 3 drops (degrees)
Calculated CO2: 57ppm

 

[sumpnfishy]

The only comment I have is ignore the "Fresh from the Tap" readings. This bit me when I started paying attention to water parameters a few years ago. Best to check the parameters after the water has been aireated for an hour or sat out for 24 hours. The water has to "gas out" (my non-technical term) before you get an accurate reading.

I spent a lot of time trying to figure out why my pH was so high but finally figured out most fish live fine in my hard well water with 8.2 pH. Fresh from the tap it reads around 7 pH.

 

[jsoong]

First I assume that you do not inject CO2 into the tank ....

The Tap water reading does not look right.
At pH of 6.5 & KH of 3 degrees, the CO2 should be ~25-30 by my calculations. Is the 167 a calculated or measured value? CO2 is pretty common in well water, so the best thing to do is to let it sit out for a day or 2 to equilibrate with room air. At which point you can assume that the CO2 is ~3, and you can then validate the pH/KH relationship. <The equation is only valid if there is only carbonate buffers in the water. Any other ions present will throw it off, so it is best to validate before trusting it.>

R/O water should have almost no ions. It should not read 14 degrees. <That is higher than your start, so carbonates must have been added, which an R/O unit should not do.> Possibilities - You don't have an R/O unit; you are reading the waste water from the R/O unit, not the R/O water; your R/O membrane is broken; or the test is in error.

Tank water actually reads much better. That is just about right for typical moderate hard water, and should be fine for most fish.

 

[Badger]

@sumpnfishy That's why I specified that it was fresh, hasn't sat yet. I'm planning on taking tests tomorrow to find out what it's like after it's been sitting.

@jsoong No, I don't yet. Those are calculated from KH and pH (only way I know how to find out how much CO2 is there)
Yeah, the tank water is the only one I thought was normal, that's why I decided to post a thread. I also thought R/O water should have nothing in it, so either I have a faulty test or the membrane is broken. Thanks for the help.

Also, the reason for this test is I'm trying to figure out if/how much CO2 I should inject, and why my water turns from very soft to hard after I let it sit for a few days.
I've been using http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm to calculate with. Is this good?

Edit: Yeah I somehow made a big typo on...everything. The tap water and RO water might have been mixed up. I'm an idiot. Changed it in the original post. Stop typing late at night, badger...

 

[jsoong]

Edit: Yeah I somehow made a big typo on...everything. The tap water and RO water might have been mixed up. I'm an idiot. Changed it in the original post. Stop typing late at night, badger...

The numbers now look much more reasonable! <But you changing things in the original post made my reply totally nonsensical! So any one reading this thread can skip that & read on from here ... >

Chuck's CO2 calculator is good. The equation uses generally accepted numbers. The unwritten assumption in the calculator is that the water contains ONLY bicarbonate as buffer.

Looking at all the numbers, I would conclude:
1. You have dissolved gases (CO2 likely) in your well water, which is why the pH comes out low & rises after sitting out. I would aerate the water in a bucket to equilibrate the pH before doing a big PWC. <You are prob OK doing small 10% changes straight out the tap, some would not chance that.>
2. Your R/O is working. It dropped the KH from 14 to 3. <Although I would expect a good unit to take it to near zero.>
3. Your tank water is tap, right? The KH dropped a bit, prob from the driftwood. No biggie. Looking at the tank number, I think you should use tap rather than R/O for your tank. <R/O water has no buffers, & you need to add buffer to make pH stable. All that work is worthwhile only if the tap is really bad.>
4. Your equilibrium CO2 measures too high for a pure bicarbonate system. There is likely some other buffer present. <Well water also commonly have nitrate, phosphate, silicates, etc.> If you are going to inject CO2, you might want to use a drop checker for greater accuracy. OTOH, the 9ppm in the tank isn't too far off, you can guess at the actual CO2 level by looking at the pH drop. You can't just read off the CO2 level on the charts though.

 

[Badger]

All right, that's all good to know, thanks. I guess I'll have to read up more to fully understand. So much to know. Thinking about more than bicarbonate as a buffer helps, though. Now to decide if I want to inject and if I want to make a drop checker...

 

[Badger]

For anyone who wants to know, the reading on my tap water after about two days is about the same as my tank water.
pH: 7.6
KH: 13 degrees
Calculated CO2: 10ppm
Phosphates: 0.25ppm

Update: Nitrates from this water are about 5ppm. Do these throw my CO2 calculations off by much?

Hopefully my testing will help someone else understand this stuff better too.

 

[jsoong]

You have nitrates AND phosphates (plus whatever else you did not measure) .. that is throwing off the calculations a bit. The calculate CO2 should have been 3 or 4, the 10 ppm you get would indicate that the pH is lower than expected for the KH. <Should be 7.9 in a pure HCO3 system.>

But it is not off by much, so I wouldn't sweat too much over it. <CO2 injection is more or less an approximation anyway. Anywhere from 10-30 ppm is fine as long as it is stable, so it is no big deal if you are off a little bit in the actual number.>