Thanks for all the info Yabbies that has been a great help.
I checked the water this morning after adding the 3ppm of salt yesterday. It looks like the PH has jumped from 7.0 up to 7.6 overnight. Should I be concerned or is this normal when introducing salt? I have some hydrochloric acid, i am considering trying to lower the ph back down to 7, would this be a good idea?
Ammonia has slightly increased to 1.5 and still no readying for nitrites or nitrates...
One other fish has been looking quite unwell, doing a bit of upside down swimming, i was going to pull him out this morning but thought I would give them until the end of the day to see if he can get back on track.
Yep... I've found pH will tend to go up a bit when you add salt. I salted my 600L Murray Cod aquarium to 3ppt on the weekend and the pH went from 7.0 to 7.2
Don't add the acid directly to your system, it will more than likely just drop the pH for a couple of hours and then it will bounce back up to where it was.
You would be better off treating any top up water to a pH of 6.0 a few hours, or the day before use, this will consume any carbonates in the water, which buffer the pH up. Overtime this method will gradually reduce the carbonate level in your system water and the pH will come down.
When you add the pH 6.0 top-up water to your system it will slightly drop the pH in the system, so be careful not to add too much at one time. If you have to add a fair amount then do it in stages, maybe 1/3 at a time and a couple of siphon F&D cycles apart, or about 20-30 mins. Top-ups should be done regularly to avoid adding too much at one time. Also, you shouldn't drop the sytem pH by more than 0.2 or 0.3 per day.
With the Amm at 1.5 I'd be contemplating a 1/3 water change. If you have some "aged" water, or rain water you can add to the system. If not I would get some de-chlor from the pet store and dechlorinate enough tap water for a 1/3 water change. Don't feed the fish, or add any more Seasol, or anything that may produce Ammonia, until you've managed to get the Ammonia down. Fish can go days or weeks without food.
It's adviseable in the early days of running a system, especially if you are cycling with fish, to have a supply of aged, or rain water, ready to add to the system in cases like this. It's as easy as buying a blue plastic 205L drum (cleaned and fish/food safe), cutting the top out, fitting a small pump (500L) and a spray bar, see pic below, and aerating the water for 24hrs. If you can place the drum in full sun it will work even better.
Also, make sure you have plenty of aeration in your FT, especially when your Nitrites start to show up.