Good point from Ogre44. Kids often think they will help the fish by giving them this or that.
But aside from that issue, to the pH. Never use chemicals or substances to change the pH, this usually does more harm than good. Water chemistry is a very involved topic, and there are a number of factors that interact. While testing pH can alert us to a possible problem, there has to be a lot of testing and thought given to resolving it.
The pH of an aquarium will normally lower over time as the water acidifies. This is caused by the breakdown of organics creating CO2, and carbon dioxide creates carbonic acid which lowers the pH. The hardness of the tap water, primarily the KH (carbonate hardness) or Alkalinity, also impacts this, as the higher the KH the more it "buffers" or resists a change in pH. All of this has to be known and considered before attempting to fiddle with the pH, as the fluctuations back and forth can severely stress and even kill fish.
The green water is another issue, though it may be related. A lowering of pH from 7 down to 6.2 will naturally be a part of organics, and these can feed unicellular algae which is the cause of green cloudy water. The green water is absolutely harmless in itself, though it may be indicative of other more serious issues. A partial water change, or several, will help to rectify this; but Boredomb's advice on sudden changes is well taken, and this must be avoided.
How often are partial water changes made on this tank, normally? And how much of the tank volume is changed? Which water conditioner? And are any other additives/substances going into the tank aside from the conditioner?
What is the tank size, and how many and which type of fish are in it? Are there live plants?
The above info will help us to sort this out for you.
Byron.
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