Restocking A Slightly Acidic, Softer Water Planted Tank

I definitely don't have the ability to store more than 10l of new water. It pretty much comes out of the tap and goes into the tank. 10l at a time, dechlorinator added as the "in" bucket fills up. I lug "in" bucket halfway across the house, up in my step and slowly pour it into the tank so as not to disturb the substrate.

The only new substrate is the play sand, which was washed till the water was clear with removed tank water (in the "out" bucket). The increase in pH trend started before the sand went in and didn't seem to be affected by it. But I did wash too much sand and have kept hold of the excess.

The fine gravel substrate came out of the tank, was sieved and then went back in.

I think I might have to buy another pH test kit. At the moment I have my Seneye and a 6 in 1 test strip (which I think is pretty awful!). So I'm struggling to get a reliable reading on my tap water.

In the past I have added calcium to the tank in the form of calcium nitrate. But not for many months and I have changed a lot of water since then.

How accurate are the hardness tests? I ask because the difference between the two readings is 3 drops of reagent for GH and 1-2 drops for KH.

And lastly, I have to say thank you for all your support that I was doing the right think micro analysing my water parameters. But I caved. This afternoon a dozen five band barbs joined my tank.    They are a similar size to my harlies at the moment, so I think they are a total of about 36cm.

Thank you @Littlefish 

Pictures should really be put in the forum rules 

 

I've just ordered / reserved some kuhli loaches from my local MA. What are thoughts on 14 kuhli loaches with a pair of kribs in my 240l planted tank? I intend to include some more caves in the tank, but my main concern is that if 14 kuhlis is a good number for the tank, is it too many with the kribs?

Would that many substrate "hunters" be too stressful for a breeding pair of Kribensis?

That is exactly what I am worried about @Matt. The kuhlis are staying on the shopping list, it is just whether I could alleviate the problem by having fewer kuhlis. If so, how many?

Or do I need to take kribs off my shopping list? They would be the last fish to go in the tank, so are a way off.

How much could I influence the kribs territory? Would a strategically placed cave help, or would they be more likely just to pick a different one? That said, my two current kuhlis seem to be enjoying the whole of the tank. The refurb seems to have made it all more to their liking.

The other issue, that has me thinking twice about getting even the kuhlis this weekend is that the pH in my tank is still going down. The maximum readings over the last week have been (all at 10pm):
(Barbs introduced Sunday afternoon)
Monday: 8.06
Tuesday: 8.16
Wednesday: 8.03
Thursday: 8.03
Friday: 7.96
Saturday: 7.92
Sunday: 7.92
Monday (today): 7.76

At which point do I start to worry and what can I do about it? The pH of my tap water is approximately the same as the tank pH today.

Carbonates are measured by KH. I don't think the bacteria themselves have much impact on the amount of carbonates, though they may use more when increasing the numbers of them; when they are just keeping the population stable (some dying, new ones replacing them) there should be little impact.

In a cycled tank, there are undetectable levels of ammonia and nitrite but nitrate is constantly being made. This is acidic - unless something stops it, it lowers the pH. Other chemicals produced by fish, plants, micro-organisms of all sorts are also acidic. These chemicals are waste products, hormones and so on - remember that fish communicate with each other by chemical signals.

When acids, bases and salts dissolve in water they dissociate into ions.

Carbonate is CO₃^2- , carbonic acid is H₂CO₃ It dissociates like this:
H₂CO₃ -> 2H⁺ + CO₃^2-

and it can also lose just 1 hydrogen ion to make bicarbonate
H₂CO₃ -> H⁺ + HCO₃^-


The reactions above are reversible; they go backwards as well. In fact they are continually going backwards and forwards but there are always the same number as H₂CO₃ and as ions.
However, if something happens to disrupt the balance, things change. Add more H⁺ to the right hand side and it pushes the reaction over to the left. So when hydrogen ions are made by nitrate production and all those chemicals secreted by things in the tank it converts more carbonate ions into carbonic acid molecules. This reaction removes the free hydrogen ions from the water so the amount of them stays constant and the pH stays constant.

With low KH there isn't much carbonate so all of it ends up being converted quite quickly. Once it is all converted to H₂CO₃, there is nothing left to remove hydrogen ions so they build up and the pH falls.
When the KH is high, there is so much that it would take months for it all to be converted. And the fish would suffer from Old Tank Syndrome well before all the carbonate had been used.

Water changes remove H₂CO₃ and put more carbonate in the water. Even with very low KH, regular large water changes are enough to stop the pH falling.




EDIT TO ADD

When carbon dioxide builds up in the water it forms more carbonic acid. This pushed the equation from left to right, which means there are more hydrogen ions so the pH drops.

So the water conditions started to stabilise. I still have the daily variation in pH, but it is back to reflecting the lighting regime. Ie pH at its highest at 10pm, when the lights go off, and at minimum at 7am just before the lights come on, with a noticeable dip during siesta time.

I did a 40l water change last night and changed the slide in my Seneye. With hindsight, I shouldn't have done these at the same time. There was a slight jump in pH, which I think was due to the slide change. I don't think it would be noticeable with a standard / liquid test, because it's either side of a colour reading. (Ie, both readings would look like 7.8 )

So off to the fish shop I went this morning. Sadly the kuhlis that I was going to get didn't survive the lfs quarantine. 🙁 So no more kuhlis. So I decided instead to get the next fish on my list - neon dwarf rainbow fish. 9 little beauties are currently acclimatising. Hopefully I've got 5 female and 4 males. In the shop tank it was easy to see the difference between the red and yellow fins. But not once they were being caught and put in the bag. So I guess I'll have to wait till the tank lights come back on to check. In the dark tank , they already look like they're starting to colour up, but I'm not sure that's just a trick of the light.
 

Great to hear that things are settling down and that your rainbows are settling in so quickly. 

After a bit more research, I don't think emperor tetras would be suitable for my tank.

But I've had a look at sparkling gouramis. Any thoughts? @Matt I think I read that you've had them in the past?

In terms of space in my tank, I'd be able to add about a dozen fish that size. But I read they prefer smaller groups, so what would be an ideal number? Not totally sure they're the right fish, but at the moment they seem to 'tick all the boxes'.

Thanks @Matt . You're right, I do want a shoaling fish so sparkling gouramis won't be right. That's a shame, they looked nice fish. I think they'd get a bit lost in my tank though, if they don't shoal.

I have looked at several tetras - penguins, Black neons and x-rays. I'd not looked at rummy nose until you suggested it. I'm not sure rummy nose are the colour I'm after - I have a lot of red in my tank. 

The thing that's making me hesitate about tetras is that they're all south American fish. The perfect fish would be an Asian one, as that's what my current shoaling fish are.

But I've not completely ruled out tetras yet.