Well, Ph isn't too much of a concern if unless you have things like Discus or are want to breed a certain species, or create a paticular biotope. Most fish adapt very well to Ph levels.
As for nitrates, nitrites and ammonia, your best bet is to do a large water change. You had a 15L tank as I saw in the last post so I'd reccomend a water change of maximum 10L. This should lower the Nitrites slowly but surely, also like I said, get your stocking in the tank right.
Maximum of the 4 white clouds and you can get away with the small goldfish if you're changing the water every 2-3days. A 10L water change each time. Be sure to use a dechlorinator on the water so you don't harm your fish.