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I'm known as the freshwater nerd on the local reef club forum, so I had someone ask me this:

His friend has a planted tank with co2, and he can't get his ph lower than 7.4. He'd like to lower it a little bit for his Cardinals.

He's done water changes with ro water.

I assume he's made adjustments to this psi. Other ideas for him?
 

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If he's using RO water, and has co2 on, and pH is 7.4 then something is either missing from this story, or his pH test kit is worth less than a peso (or was it dollar?...)
 

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If he is telling you everything then either he is using really small amounts of RO water or he messed up and is not actually using RO water.
 

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All of you could be wrong. Depending on what he is using for substrate would answer the alot of questions. I use a gravel / sand that has shells in it that make up some of the gravel / sand. If this is the case than the shells or small amounts of coral could keep the ph at this level. I had this happen to me in a tank years ago and it took me for ever to figure it out. So I would find out what type of substrate he is using. If he is using a black beach type gravel / sand ( has different colors in it other than just black ) its probably causing the ph to stay higher than in should be. Just my two cents worth hope it helps out. If not sorry... I tried.
 

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+1 to what zid said. *IF* the guy is telling the truth and/or has the story straight in the first place. If everything else can be ruled out, then it is DEFINTIELY something in his substrate, or rocks/shells/etc upping the pH.
 

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First thing I can see is that he is not injecting enough CO2. Whats his BPS? and what size tank.

Good point though about the substrate, howver, that would only buffer the waters carbonate hardness. You can still force the pH down with more CO2.
 

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Please read more carefully next time.

All of you could be wrong.
something is either missing from this story, or his pH test kit is worth less than a peso (or was it dollar?...)
That still sounds 100% correct to me. What could be missing from the story? Buffering substrate, rock, the fact that he dumps baking soda back in, etc. etc.
 

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simple question to ask: What is your KH/GH?? that should tell you about other factors Substrate/ decorations ect...,if he is using RO...
 

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First thing I can see is that he is not injecting enough CO2. Whats his BPS? and what size tank.

Good point though about the substrate, howver, that would only buffer the waters carbonate hardness. You can still force the pH down with more CO2.

You could force the pH down for a short time. If he is adding Co2 durning the day and not at night the water is going to rebound. If you check the alkalinity and the hardness it should help paint a better picture of whats going on. You can add all the Co2 you want but if the substrate you are using is coral or shell based you are going to have problems with your pH.
 

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Ok, I'm the friend of the guy who has the tank.
His test kits are fine. The ph has tested at 7.4 on different kits and he also has a ph meter as part of his co2 system. And yes, the meter has been calibrated.
He has Flourite over a small amount of standard aquarium gravel for a substrate. I haven't done an alkalinity test but he said the LFS has measured it at 4.0 twice.
I'm not sure on the hardness level but I think it fell in the "moderately hard" range according to his test kit.
It is a 75g tank.
He is running co2 at about 2 bubbles/sec. This keeps the ph at 7.4 but it does not go lower if he turns it up.
His plants are doing very well but he is losing some fish. The fish have no obvious signs for there demise. He has lost Cardinals, some other small tetras, and even 3 corys.

Any thoughts or suggestions?

Thanks.
 

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He needs to address TDS if he wants his Cardinals to benefit. If fish were affected one way or the other by the pH shifts that result just from running CO2 they'd all be dead in everyone's tank who runs CO2.

I don't the the pH shifts from CO2 are the answer to the question why are the fish dying- there's something else going on.

Is he using anything else to try and modify his pH? (Not a good idea if he's been using chemicals.)

If you guys have reefing background then you know how to reconstitute water and the need to establish and maintain stable parameters. Stability needs to be his goal in regards to his water parameters. All of the fish mentioned should be able to adapt just fine to a pH of 7.4 and moderately hard water- I keep all those fish in my Florida limestone well water with moderate hardness and a pH of 7.2.

Has he ruled out other possibilities for the fish deaths? Infections/diseases, parasites, organic wastes, temperature fluctuations... how long has he had the fish, did he maintain strict QT before adding new fish... all those same things that are necessary with SW tanks...?
 

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Yeah, I know the fish can adjust to 7.4, I just thought it was odd that his ph won't go any lower than that.
No he is not using any chemicals to adjust the ph, espcially not the phosphate based buffers.
Unfortunately he is not a big QT guy. So I am sure it is likely the fish are dying something that was introduced, but it is odd they seem to show no signs of distress and no symptoms. The tank has been up for 5 months, but most of the fish are recent additions. Ammonia always tests at 0 so that is not the issue.
I was just curious if there was any reason his ph won't drop that might also affect the fish. What TDS should he be shooting for? I only have an inline meter on my RO but I'm sure I can get hold of a hand held meter.

Thanks
 

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His pH probably won't drop b/c his tank is too buffered with kH. Pretty common with us in FL b/c our water comes straight from our FL limestone aquifer.

IMO his fish are most likely dying from acclimation stress and issues they're bringing with them into his tank, not his water parameters.

If he's set on wanting to recreate "natural" conditions for his fish, then he should probably look up the species he's keeping on www.fishbase.org and www.planetcatfish.com and try to find a happy medium based on the TDS recommendations for his species.
 
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