Hey Zorfox I really appreciate your time to answer that for me.
I had to read it a couple of times, to make sure I understood it all, the "molecular formulas" are what always throw me off, where ever I read about fertilizers on here or Google, everyone says the molecular formulas lol, I guess I will just have to learn them to make life easier. But thanks for giving such a good answer that I could understand, I am still confused about the correct dosages etc, but if you reckon it sounds good I will stick to what they said.
I looked at the calculator and found the solution part, I still don't understand 100% what ppm I am aiming for, I presume the calculator has this set to the correct limit, for example KNO3 its telling me how to dose for 7.5ppm, it also lists the measurements of how much to add to a 500ml solution, I presume I just measure them against what the website recommend.
So in this case I would measure 4tsp of Potassium Nitrate and measure to see if it comes to around 30.572g.
Also you said "Not dosing the potassium nitrate may leave you potassium deficiency" thanks for telling me that, at least I now know what could be going on with the pinholes in my plants, I use the API nitrate test kit from the master kit, my pond, tap water, tank, is always 40ppm... I also know my tap water has 40ppm nitrate I believe as I looked at my water report, but like you say it should go down, which is why I always am mind boggled, because they never do! So I will be sure to read that link you posted about calibrating the test kit.
I also have no idea why they would give me epsom salt, is there a way I can test to see if it salt, I don't see why they would give me that lol... are you sure I've been ripped off? if I have I better look at getting the magnesium elsewhere.
This is a bit more complicated than it should be.
London's tap water tends to be fairly high in NO3, so many opt that out and add some K2SO4 to replace.
So for a 90 liter tank:
2-3x a week:
1/4 tsp K2SO4
I'd add the MgSO4 to the trace mix and make a solution using about 4 tsp of CMS+B and 2 tsp of MgSO4, then dose this at about 5mls 3-4x a week.
That is it.
Feed fish, add algae eaters, do water changes and really focus on good CO2.