What water source are you using? The only reason for there being ammonia in a tapwater supply is if your water company uses chloromine to treat the water, if this is the case then you will need to use a conditioner that neutralises chloromine and a product such as AMMO LOCK to detoxify the ammonia, alternatively you could run the de chlorinated water through a established biological filter to remove the ammonia before adding it to the tank.
Rainwater poses the problem that in some areas it can be heavily polluted and also has no measurable hardness so is very unstable, when using rainwater you must re mineralise it with the essential trace elements and buffers just like you would with R/O water otherwise your tank will suffer from huge pH swings and could crash altogether.
Personally i wouldnt recomend playing about with the pH unless you are very sure of what you are doing and have a good understanding of water chemistry. Unless you plan on keeping delicate fish that require soft acidic water to survive then a pH of 8 will be fine as most common tropicals will happilly live in it.
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