pH problem

[arjay]

I have an AR620 aquarium which is regularly maintained and not overstocked (9 cardinals, 5 pristellas and a honey gourami). For the last 6 months pH readings have been below 6.0 (I only have a test-kit with reagents which doesn't read below 6 so I can't be more accurate) even though tap water here is around pH 7.8 and I have another established tank which has a pH of 7.6. After 1/3 water-changes the water in the tank returns to its low pH within a short period.

I don't overfeed the fish (2 feedings a day with them getting no more than they finish in around 45 seconds) and CO2 isn't a factor. The filter is well maintained (I'm still using the filter-box which is located on top of the tank) and dead leaves are removed from plants when I notice them. I checked ammonia and nitrite and both were in the safe zone.

The fish which are currently in the tank are fine but Corydoras can't cope with the conditions and begin gasping at the surface and can't settle on the substrate, they sort of float when they're at rest. Additionally, the leaves on the Amazon swords in the tank have begun dying off (they seem to rot at the petiole) but the Cryptocoryne are fine.

Anyone have any ideas on what could be going on?

[DiscusEden]

What's the kH like?

I've had similar issues, caused by a low kH in my water (in the hills), allowing the pH to crash in a small period of time.

Probably also worth testing the gH.

Just a thought.....

[arjay]

I really should have checked that too - will do so now (I'm also in the Adelaide Hills btw)...

If the kH is low is there any reason as to why it'd be an issue in one tank and not the other when most of the variables are the same?

[arjay]

The carbonate hardness is below 10ppm.

[DiscusEden]

I'm just finding out about this myself, so happy to stand corrected, but from what I can gather, the low kH doesn't lower the pH, but it makes the pH of the water more prone to fluctuations.

So.... the amount of waste produced by the fish causes a major drop, which shouldn't otherwise occur. The way I am (trying) to deal with it is to add shells and coral sand. The sand is more effective (smaller grains, larger surface area, more easily dissolved).

I have the same issue with it only affecting one tank, which is only half stocked. I had CO2 on the affected tank, the CO2 dropped the kH, which caused the crashes.

I got the coral sand at Reef & River.

Oh, the other question is what brand of test kit you have, and whether you're using Seachem Prime, which can interfere with other brand's tests.

[arjay]

I read through the topic which you started regarding low pH and did a little research. I'll pick up some crushed coral over the weekend and see if it'll bring the KH up to a satisfactory level. Thanks for the help and good luck with your tank also...

[DiscusEden]

Thanks arjay,

Let us know how you go?