PH measures the degree of acidity or alkalinity present in the water of your aquarium.
On the pH scale 0 is the most acidic, 7 is neutral and 14 is the most alkaline.
Most fish happily tolerate anything between 6.
4 and 7.
8 but there are some exceptions.
pH is a logarithmic scale, in simple terms this means that a small change in value indicates a very large change in acidity or alkalinity.
It is much more important that you maintain a consistent pH.
While a fish may prefer a pH of 7.
2 it will most likely be perfectly happy with a pH of 6.
7.
However that same fish will be really stressed if it has to endure a pH swing of 0.
3 in a day.
Most aquarists deal with the pH issue by keeping it as constant as possible and getting their fish used to living in that environment.
Constant means that the pH should not change by any more than 0.
2 over 24 hours.
In some cases breeding only takes place at a particular pH value.
If you are hoping to breed a certain variety then you will need to research their preferred pH range.
Some species have a very tight range while others are more forgiving.
This is probably the only good reason why you might want to aim for a particular pH value.
Factors Affecting pH

• Biological filtration As you already know, filtration is essential in a well stocked aquarium.
  Biological filters reduce pH because they release carbon dioxide into the water.
  
• Carbonate Hardness Carbonate hardness or kH is a measurement of the carbonate hardness of your water.
  A high kH (i.
  e.
  the harder your water) the better is the water able to neutralise added acids without significantly changing the pH.
  This is known as your water's 'buffering capacity' a good kH is above 4.
  5.
  

Adjusting pH Frequent water changes (which you should be doing anyway) are the best way to keep the pH up to an appropriate level.
As stated above you should acclimatise your fish to live in the ph that you provide but if it is necessary to change the ph for any reason then here's how.
How to Lower the pH

• Filtering water over peat
• Add bogwood to the tank
• Inject carbon dioxide CO2
• Use a commercial acid buffer
• Water changes with softened water or RO/DI (Reverse Osmosis/De-Ionized) water

How to Raise the pH Raising the pH makes the ammonia in the water more toxic to your fish so this should be done with great care.

• Aerate the water, to drive off excess carbon dioxide (CO2)
• Filter over coral or limestone
• Add rocks containing limestone to the tank or use a coral sand substrate
• Use a commercial alkaline buffer

Conclusion The particular pH value is not so important, what is important is to have a reasonably constant pH value.
To achieve this the kH should be around 5-6 dH in order for the water to have enough buffering capacity.
Swings of just 0.
3 pH in a day can be deadly to your fish.
Unless you have good reason it is best to leave the pH alone unless you have a specific reason for changing it.