When you say plantbrain lol do you mean Tom Barr?
Yes. I try not to use full names online with all the security issues now. Although, his signature does have his name so...
Also are you referring to " If the tap has consistently high NO3, say 30ppm, then you do not add KNO3, instead, use K2SO4 etc."
Exactly. KNO3 is used for nitrates (nitrogen) and part of the K+ (potassium). If nitrates in your water are already in range no need to dose it.
So in my case if I do have high nitrates, I can just buy some K2SO4 work out how much I need to add to my 500ml solution, then carry on my EI and hopefully the plants will be getting there potassium.
If this is the case do I have to worry about any fish/shrimp coming to harm with K2SO4?
Not at all.
I also forgot to ask you yesterday, I looked at the calibrating thread you linked, and for example my tank always shows 40-80ppm Nitrate its so red I cant tell which ppm it is but 80ppm sounds way to high for tap water, but if I calibrated it and tested a cup with 25ppm and the test kit shows red, its still going to be hard to determine my reading, as in theory if I tested a cup with 10ppm nitrate I could possible still get a red reading, I will certainly give it a try though and see what happens.
With API kit below 20ppm should be a brown color. If you know the water is set to 10ppm and your'e seeing red, replace the nitrate reagent. I suspect your problem is the high nitrates in your water. When the nitrates get up to 40ppm I can't tell the difference anymore. It all just seems red to me. You can "cut" the sample with 50% distilled water and double the result if it's too high to read.
I don't know if you can recommend me any good test kits ?
I use API master kit and a few other single ones. For an accurate KH I use Salifert which has a very narrow range. Other than that I just use the inexpensive API tests. Sure you can get a bad reagent at times. That can happen with any brand.
Once you get things dialed in you really shouldn't need to test much anymore. So I wouldn't bother spending any money on expensive test kits.
MY water report says: Nitrate 50 mg NO3/I
That's the reason all your results are red lol. As plantbrain said. Keep it simple and use K2SO4 (potassium sulfate) in lieu of the KNO3 (potassium nitrate).