Bert,
I did not know you were in Austria! I have no idea how your location handicaps your ability to lay hands on aquarium products, so please forgive my presumptions going forward. Aquatic gardening has been long practiced on the Continent. You are able to obtain fish, plants, test kits, water conditioners, equipment, and so forth. Are there other hobbyists also shopping for such things in your neighborhood that you can speak with? Potassium sulfate should not be a rare commodity where you are.
You are able to purchase Easy Life Ferro. Easy Life has a complete line of aquarium plant food products. You can get pretty much everything you need, including potassium, from Easy Life. They have a website you can go to and see their offering. The shop where you buy Ferro should be able to get you any of these products.
Yes, 7 ppm chloride is quite a bit. To be sure, there are many plants well-known in the hobby that are native to waters having this much chloride, and some plants are more adaptable than others to salt content. However, in soft acid water typical of tropical streams, chloride should be nearly undetectable. A broad spectrum trace supplement would almost certainly have chloride in it, or you could toss a teaspoon of KCl into the tank monthly. That would be all the chloride your Anubias and other plants will ever need. Chloride is an essential micronutrient. It is easily taken up by the plant and is very mobile within. It is important for osmoregulation and as an enzyme cofactor. It is a metabolic agent and not grossly structural. Structural stuff (like N and C and S) is not intrinsically toxic and you can dose for luxury uptake as a rule with few concerns. Metabolic agents such as enzyme cofactors can be toxic in excess, and most of these are metals such as zinc and copper that are required by plants but can be overdosed (metal toxicity). As a matter of best practice, any ion that is not assimilated structurally should only be provided in just the quantity required and no more. I am not aware of a well-documented toxic hazard of chloride to specific types of plants. But unless I was keeping an estuarine brackish community, I would not think of having anything close to 7 ppm Cl. An unnecessary risk is not justifiable when the stakes are high.
I assume you have been using KCl regularly for some time. Are you also using a nitrate supplement regularly? Is it potassium nitrate, KNO3? Same with phosphate - is it a potassium salt? In fact, let's begin at the beginning. How big is the tank, what do you put into it, how much and when. Test your source water for everything you test your aquarium water for, and let's see what we are working with. We're stumbling in the dark without this info. But I will say this: with all the iron you appear to be putting into your water, the problem is not in the actual amount of iron present. Yellowing and lightening of green color is a problem with chlorophyll production, a syndrome called chlorosis, that can be caused by numerous conditions. One common cause that is very similar to iron deficiency is potassium deficiency.
I use test strips to get fast approximations of certain parameters. They are useful for this purpose only and should not be relied upon to obtain actual data. When you get really wild results like that, you should suspect something is amiss until confirmation arrives. It is difficult to have 100% confidence even in reagent type test kits where better accuracy is an implicit goal. This is due to shelf-life limitations, subjectiveness in color and turbidity valuations, poor lab techniques (I can still hear my chem professor chastising me all these years later when I ask myself "how many drops was that now?"), or just plain pretentious claims for poorly designed tests that some think are good enough for amateur hobbyists. Be careful to get the best result you can from a test, but also bear in mind that that was the best result you COULD get from that test.
Looking forward to hearing back. Leave nothing out.