Problems with new tank setup (2 weeks old)

Around 2 weeks ago now I ventured into fish keeping again, last time
was over 5 years ago. I am starting with a small tank, and maybe move
up in size. The tank is around 13-15 litres.

3 goldfish, nothing fancy, at the moment there are no plants. I am
using a Fluval 1 "Plus" Underwater filter. I started my tank off with
tap water, left over night, I also added Tetrafin Aquasafe into the
water to make it safe for the fish.

This was 2 weeks ago. Within about 24 hours the tank went quite
cloudy, kind of a green/grey colour. After some research, as I have
never had this problem in the past, I thought it may be a case of "New
tank syndrome".

Just now I tested the PH and also ammonia levels - ammonia was at
1.5mg/l and the PH was around 8.5. These I think are a little high.
Should I just wait it out for another few weeks - after all the fish
seem to be feeding fine and appear to be "happy" - or should I part
change some of the water, or would that just make the problem go on
longer?

Thank you in advance,

Jon