Hi again ... Thanks for the Chat yesterday. Have you considered this article: http://www.directpoolsupplies.com.au/webcontent34.htm (click specifically on maintenance of your salt cell) There are lots of good tips here for the pool owner.

I see that this manufacturer wants 4000 ppm salt (4%) minimum to 7000 ppm salt (7%) maximum which is above the standard here in North America. You are in Australia? According to the manufacturer, your cell may be at the end of its life.

Sudden low filter pressure could be a broken lateral in a sand filter (you would probably have sand in the pool) or a hole in a paper filter (no indicator other than low pressure).

Here are some useful numbers for your 4 X 12 X 30 foot pool:

1440 cubic feet or 40.776259104 cu meters
1 cubic foot = 7.5 gallons or
1 cubic foot = 0.0283168466 cubic meters.
1 cubic meter = 264.172052 US gallons.
1 cubic meter = 1000 liters ( you probably know this stuff though)

I calculate your pool to be about 10800 gallons. The actual gallons per foot is really 7.48 (10771.2 in your pool) but we use 7.5 gallons per foot.

One inch in your pool is 225 gallons or 851.71765 L. This will give you some idea about what chemicals you may have to add with one inch of make up water.

If you were in NJ, and a commercial pool, you would have to have a a 8 hour turn over rate with a filter = 60 square feet and your flow rate would be about 22.5 gallons per minute.

Please keep your water balanced with the proper amount of Calcium (about 350 ppm) your pH at 7.4 - 7.6 your TA around 100 (80 min and 120 max) and your salt per manufacturer recommendation, in this case about 5000 ppm. Cyanuric acid around 80 ("Sun screen" for chlorine) and of course Cl around 3 to 4.

I hope you have found this information helpful. Enjoy your pool, while we shiver here in the North East.