#### saqib

Hi Everyone
Appologies if this has already been addressed somewhere, ive been reading a lot through the various posts on this forum and/or other forums as well.
Needless to say its getting far too confusing for me and need your help.
For simplicity im going to use the example of one of my tanks here however do note that i need to apply the same logic on 5 other tanks

For now im using RO water only for top off. I have to replace about 12 litres of water everyday to compensate for the evaporation that occurs.
12 litres = ~5% of the water in my tank
My source water has significantly high amounts of TDS (250-300+) and PH is around 8. The water is generally hard with a lot of calcium in it.
My intent on using RO water for top off is to maintain the TDS levels in the aquarium and not letting them get too high by adding more everytime i top off.
I'm still relying on using the source water for the weekly water changes, cant get into the whole practice of collecting RO water and then adding equilibrium, PH, Acid buffers etc....thats too cumbursome for the amount of water and tanks i deal with.

Question 1:
Topping up with RO water instead of source water would allow me to maintain the TDS levels?

Question 2:
This is where i'm getting really confused.
The source water as stated has a PH of 8, the RO water comes out at 7. Everytime i top up using the RO water, do i change the PH of the tank ever so slightly? or does it work in the same way as the TDS thing, i.e. evaporation causes the PH to rise/drop and by adding the RO water its just going back to its original levels?

Now if it does indeed change the PH of my tank, i top off 5% of the water everyday that essentially means ive replaced the tank water with RO by almost ~30% by the end of the week.
My PH would be completely out of sync with the source water then, correct?
My fear is that when the next time i do a weekly water change of ~30% with the source water, the PH was rise up and rise up significantly fast and that cant be good for my fish nor the plants.

Question 3:
Could i just mix 50% tap & 50% RO so that the TDS is halved from whats coming from the source water? Could i avoid PH swings and maintain a relatively good water chemistry that way?

#### MacZ

Topping up with RO water instead of source water would allow me to maintain the TDS levels?

Yes. As you only add to the water volume, but not to the dissolved solids.

The source water as stated has a PH of 8, the RO water comes out at 7. Everytime i top up using the RO water, do i change the PH of the tank ever so slightly? or does it work in the same way as the TDS thing, i.e. evaporation causes the PH to rise/drop and by adding the RO water its just going back to its original levels?

Now if it does indeed change the PH of my tank, i top off 5% of the water everyday that essentially means ive replaced the tank water with RO by almost ~30% by the end of the week.
My PH would be completely out of sync with the source water then, correct?
My fear is that when the next time i do a weekly water change of ~30% with the source water, the PH was rise up and rise up significantly fast and that cant be good for my fish nor the plants.

pH doesn't change with dilution in a linear fashion like hardness. pH measures H+ ions in the water, the more the lower the pH. But pH rises and falls in an exponential fashion: E.g. for lowering pH from 8 to 7 you need a set amount of H+. To lower it from 7 to 6 you need DOUBLE the amount.
But carbonates (KH) and H+ are constantly balancing each other out via CO2 from the air. So when you add RO that has neutral pH from H+ and CO2 being in equilibrium, KH drops and H+ get diluted as well. CO2 from the air will start to balance with the new levels and so usually there shouldn't be a significant change in pH as long as there is KH in the water. Edit: CO2 adds H+ when dissolved in water, so as there is less KH, more free H+ ions are in the water, hence the pH lowers to a new equilibrium.
Only quick (change takes less than 4h from level a to level b) pH changes of more than one (1.0) pH-point can be detrimental, lower range fluctuations (e.g. 0.5 points) are irrelevant and within the range of normal fluctuations in the course of the day in any tank with live plants.

Question 3:
Could i just mix 50% tap & 50% RO so that the TDS is halved from whats coming from the source water? Could i avoid PH swings and maintain a relatively good water chemistry that way?

Yes, you can mix and pH swings should be avoided as long as there is KH. Just be aware that the pH will change to a new standard level, as due to reducing carbonates to half the equilibrium between KH, pH and CO2 will end up in new numbers. They say by every 1 or 3 (? Not sure anymore. Too many numbers to keep in mind) degrees of KH you reduce the KH level the "normal" pH will drop by 0.3, provided it generally stays above 1° KH.

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