That sounds about right for a lot of the UK's water.
Hard and high NO3's, use a RO water filter if you drink/cook with that stuff.
It's fine for washing, showering etc.

You can also replace K2SO4 instead of KNO3 for the dosing, you have plenty of NO3 and the fish waste added to that ought to round out the week if you do 50% weekly changes, you might add say about 1/4 of the normal KNO3 dosing if things look okay and there's a fair amount of plant growth.

Regards,
Tom Barr