Adjusting pH in water has only one obstacle, and that is KH. If your KH is very high, it is quite hard to change your pH. If your KH is low, you can cause large pH to change drastically, which can be fatal to your fish! The trick is to maintain the right balance. KH cannot be too high or to low when you adjust pH. I have known of cases where people buy some commercial chemicals to lower pH and it crashed their pH and their fish died. They then angrily condem the manufacturers. In fact, the pH crash is due to their low KH !
pH adjustment with Chemicals
If you want to go the chemical way, the safe approach is to alter your pH by changing your kH. Instead of buying products like "minus-ph solution" or "plus-ph solution" (note: those mentioned are fictatious produces cos I dont wanna offend any brands) that are acid or alkaline solutions, look instead for KH buffer powder. When you get the KH powder, there should be an equilibrium pH value on the label. Look for an equilibrium pH value that you are aiming at.
pH adjustment with CO2
When CO2 is injected into water, a weak carbonic acid is formed that lowers the pH. Changing pH with CO2 does not affect the KH, so once the CO2 injection is removed your pH will go back to what it is before.
pH adjustment with natural materials
This is the best method for adjustment because it adjusts pH steadily (unless you dump an inordinate amount of them into your tank!).
You can lower your pH using:
dried ketapang leaves
note!: Use only those on the ground don't pluck them from the branches because it contains toxins if you pluck it. When it drops naturally, the toxin is purged in the leaf dying stage while on the tree and the connection is sealed and dried before the leaf drops.
Driftwood is also a good alternative, however different driftwoods leak different amounts of acids. Also, it is not practical to keep changing your driftwood.
peat is also good, and besides lowing pH it also softens the water. Put the peat into a filter bag so it doesn't dirty the water. The bag can be put in your tank or in your filtration system.
To accelerate the process, you can boil it like the way you make tea and use the solution by itself.
pineapple skins and lemon juice
Be careful with these.. they can crash your pH if you're not careful.
You can raise your pH using:
These are sold in packets in the fish shop and comes in a variety of sizes.
These can be picked up from beaches or from some fish shops. Be careful that you get pure sea shells that has not been laquered.