PH: aged water 7.5 versus tank water 6.6

I needed to do a 50% water change (to reduce nitrates).
It is the first time i have not gotten a PH drop by aging the water: tested tap and water aged 24h with loads of bubbles and it's the same.
Can i do a 50% change of should i spread it over a few hours?

A 50% water change should be fine...

But not if you dont normally carry out regular weekly water changes...

For instance if you normally carry out say 25% weekly water changes then it shouldnt be a problem...

But if you only carry out small water changes monthly then the large jump in water change quantity can shock the fish...

In my opinion I wouldnt be happy with such a large fluctuation in ph all at once!
Im not a scientist and am not sure of the acidic difference (Igm would be the man for the science) but there is a huge acidic content difference between the stated ph values

I would try to get the values closer together before making such a large change of water

I could be wrong (its happened once or twice before) but I would encourage caution!

If you normally do 50% water changes (even per day) then that is one thing, but if this is a 50% water change "out-of-the-blue" in order to lower nitrates (as you say) then I would recommend a different tactic for lower the nitrates initially.

Personally, I would use a mild method initially such as using JBL NitraEx (or similar nitrate removing sieves) to prevent shock.

Then followed by a few partial water changes not as large as 50% (eg 3 lots of 20% over 4 or 4 days will make a ~48% change).

On the pH, you tank has a free acid concentration that is just under 8 times the acid in the aged water.....so that is quite a difference.

However, the funny thing about pH and real fish tanks is that the water may be (I said "may be") well buffered....and even a 50% change of water may have little effect.
But....BUT.....you would need to know the buffering capacity of the water and the types of acids present to calculate whether or not the pH would swing madly or if it would stay pretty stable.
You are not generally dealing with a pure chemical mix of pure water and a strong acid.

On a related note that may be in your tank (or may not be)........I would like to know if the high nitrate and lowered pH are related:
in a tabk with low alkalinity (eg soft water) the nitrification process can cause an acid crash if there is lowered alkalinity or insufficient bases to react with and neutralise the nitric acid made by the nitrifying bacteria.

So...check your KH or alkalinity.

ian

igmillichip wrote:

If you normally do 50% water changes (even per day) then that is one thing, but if this is a 50% water change "out-of-the-blue" in order to lower nitrates (as you say) then I would recommend a different tactic for lower the nitrates initially.

Personally, I would use a mild method initially such as using JBL NitraEx (or similar nitrate removing sieves) to prevent shock.

Then followed by a few partial water changes not as large as 50% (eg 3 lots of 20% over 4 or 4 days will make a ~48% change).

On the pH, you tank has a free acid concentration that is just under 8 times the acid in the aged water.....so that is quite a difference.

However, the funny thing about pH and real fish tanks is that the water may be (I said "may be") well buffered....and even a 50% change of water may have little effect.
But....BUT.....you would need to know the buffering capacity of the water and the types of acids present to calculate whether or not the pH would swing madly or if it would stay pretty stable.
You are not generally dealing with a pure chemical mix of pure water and a strong acid.

On a related note that may be in your tank (or may not be)........I would like to know if the high nitrate and lowered pH are related:
in a tabk with low alkalinity (eg soft water) the nitrification process can cause an acid crash if there is lowered alkalinity or insufficient bases to react with and neutralise the nitric acid made by the nitrifying bacteria.

So...check your KH or alkalinity.

ian

Ian my KH was usually 2 and is now closer to 0 or 1 (API yellow right away), but please translate your last paragraph to me in simple english as I am totally lost.
On another note fish are used to frequent water changes but usually in the order of 20-30% 2-4 times a week and in my other tank more like 40% 4-5 times a week. I did do a 50% on both but added water over the course of 3 hours .

This is all about RedOx and RedOx potential (some call it ORP) and buffering (pH and RedOx).

If we look at a small part of the nitrogen cycle that can occur within the biological filter system of a fish tank under aerobic conditions, we will concentrate on nitrosofication and nitrification:

Nitrosofication…… ammonia/ammonium > nitrous acid/nitrites (simplified)
Nitrification….. nitrous acid/nitrites > nitric acid/nitrates (simplified).

Note: ammonium is an acid salt of ammonia (ammonia is a weak base);
Nitrites are basic salts of nitrous acid (nitrous acid is a weak acid);
Nitrates are neutral salts of nitric acid (nitric acid is a strong acid)

Both reactions are favoured by alkaline conditions or conditions with a reasonable alkalinity (not that alkaline conditions and alkalinity are not the same).

If there is ample alkalinity (for example in alkaline waters or where there is an alkaline buffer etc) then nitrites will be produced from nitrosofication, and nitrates will produced from nitrification.

If, however, the tank lacks such buffers (eg in soft water or water that has exhausted the RedOx potential to donate cations eg calcium/magnesium/sodium etc) then the output of the biological filter could be nitrous acid and nitric acid.

Nitrous acid and Nitric acid will tend to lower the pH.

This is quite common in tanks that have very soft water.

Now.....on your tap water..........did you do an ammonia test on it ?
Ammonia will raise pH.

ian