New tank questions (another one)

[raharlow]

Bunch of questions related to water chemistry. Below is the tank as of today, day 24ish. Ammonia and Nitrites nil (from a high of 5mg/l+) (Sera test kit).
Water source is well, with a 7.6 pH and a KH and GH of about 3 (most recent lab test)

Injecting co2 via reactor. Good flow in tank. Getting about a 1.4 drop. Starting CO2, 2hrs before lights on, lights on 11 hours, CO2 off 2 hours before lights off, 4inch round air stone on in sump at lights out till CO2 on in morning.
Have RO/DI system, all new filters. pH of rodi is about 7.8 in the bucket. Using Apera pH tester.
KH of tank is about 4 (sera test kit)

Lastly, added 6 Otos and 3 Amano's yesterday. They all did well and did a great job cleaning up a bunch of diatom algae. Can actually see my drop checker now!!

Dosing daily, to achieve weekly ppm 9.27 NO3, 3.14 PO4, 12.34 K, 1.23 Mg, .68 Ca, 2.12 CO3, 6.78 Cl, 4.87 SO4 (home brew) and micros, FE dtpa .125. Fe Gluc .125, Mn .05, Zn .035, B .053, Cu .003, Mo .0018

Biggest question is pH. What is keeping my pH high. I read that pH of RODI is inaccurate, and when I have done water changes, I'm doing about 1/3 RODI and 2/3 Tap/Well water. I thought the RODI would help bring down Tap water but I'm still above Well source at 7.6. Rocks in tank are local and mostly granite, schist and quartz. No bubbles on acid test. Does a pH swing from 8.1 to 6.7 (CO2 injection) hurt the fish. Was hoping this tank would be acidic for South America but maybe I need to rethink that. I may add a bunch of Catappa leaves. My KH and GH are both in the 3-4 range. Plant growth has been good with exception of Buce melt. Eichhornia diversifolia, Hygro sunset, AR Mini, Mayaca fluviatilis all growing fast.

Anything else you can see based on my information. Thank you for your thoughts!!

 

[_Maq_]

Generally, it's (bi)carbonates which push pH to higher values. If your KH is "in the 3 - 4 range", that's it.
Concerning your RODI water, this is strange. Yes, measuring pH of such a water is unreliable. However, it should be acidic (about pH = 5) due to dissolved CO2.
Generally, if you blend the same amount of RODI water and water with pH = 7.8 and KH = 3.5, you cannot expect that the result would be much acidic. We're not in the realm of linearity but exponentiality. Ionic strength of RODI water is very very low.
Disclaimer: I've never injected CO2. No experience with that. Not very sure what amount of CO2 is needed to push your water into acidic. I know the theory but reality is often different...

 

[raharlow]

Thanks. Your response prompted me to test GH and KH of my rodi. Essentially both zero with Sera test kit. So, hmmmmm. Decided lets calibrate pH meter again. Was a little off, so now calibrated numbers are very different. Rodi is closer to 6.8ish and who knows what my tank is without CO2. Currently it was 6.5, lower than it had been so I'm guessing that without CO2 its mid 7's which I can deal with. Will wake up at 4am tomorrow and test again! Will come back with better numbers tomorrow. Newbie mistake and I'm not really a newbie. Ugh!!

 

[dw1305]

Hi all,

Essentially both zero with Sera test kit. So, hmmmmm. Decided lets calibrate pH meter again. Was a little off, so now calibrated numbers are very different. Rodi is closer to 6.8ish

It is really difficult to measure pH in <"very low conductivity water">, even with an expensive two point calibrated pH meter. It is one of the reasons I'm not keen on pH (measured with a pH meter) in soft water as a measurement to base decisions on. <"Aqueous Solutions of Salts">

You can raise the conductivity of your water sample with a <"neutral salt"> (like NaCl or KCl), but they have to be pure, <"sea (or table) salt"> contains too many bases to work.

cheers Darrel

 

[raharlow]

Understood, which is why I kind of discounted the pH of the RODI. With my tank at a KH of 3 I kind of expected it to be lower in pH, and since my meter was in need of calibration, it probably is lower. Thanks

 

[dw1305]

Hi all,

With my tank at a KH of 3 I kind of expected it to be lower in pH

You are right on the boundary of the stable pH from the <"carbonate ~ CO2 ~ pH equilibrium">, which might account for the higher than expected pH value.

cheers Darrel