Iím getting a little confused here. In your first post, you stated that you have 8 dGH from your tap and now you have zero? Is it that variable all the time? If you are at zero now, it would indicate a water softener that is malfunctioning. You may want to talk to the owner/manager to find out what is going on. If a softener is involved, there should be virtually no sodium coming through. Most of it is washed down the drain following the ion exchange process.
Another point of confusion: you have about 17 gallons of water and are adding 6-12 ppm (1-2 tablespoons of Equilibrium) of Mg a week (plus any from your variable tap) Öand that is not enough magnesium!? It is certainly enough potassium. With 40% w/cís, that would accumulate to 15-30 ppm Mg (with no plant uptake) in about a month. There is no way your plant mass is uptaking more than about 3-5 ppm Mg a week and that is being generous.
To satisfy yourself, you should determine your Mg levels. Buy APIís Ca test kit and, with your GH numbers, you can then calculate Mg levels. We can tell you how to do this if you need help with it. I have a suspicion that your ferts are out of whack after adding so much equilibrium.
I am still more inclined to think that the substrate is heavily involved in your issue and would suggest at least trying @DaveKS
suggestions, in the meantime.
Iíd be interested in your substrate venting technique, if you wouldnít mind detailing it.
I don't know anything about my water. We don't have individual meters so the building pays our water and we pay it back through our HOA. We have water shut offs to deal with plumbing (without detail) about once or twice a month. Sometimes, I get black stuff coming out for a second or two, often I do not. I get no bill, nor do I have an account with the water company. It would not surprise me if my water varies. We have maintenance issues elsewhere, and with plumbing, so if there is a water softener, my guess is that it isn't maintained the same way it would be if it was owned by me, or anyone else with an aquarium.
I cannot answer your questions as I don't know enough. I could go out and buy every test kit but usually the only way something is figured out is when someone else had the same issue. I don't mind buying them but the last time I had a weird issue, I just switched to RO water. I had a garage back then though.
I don't necessarily believe I have a deficiency of anything. If I had to guess, I imagine I have excess something.
I don't dose Equalibrium just because, I do it because it is the only thing I have found that keeps my stems from turning black and separating. If I don't do it, my tank essentially continues to have plants separate over and over. Then I have stems that are constantly trimming themselves, never making any progress, and then I quit.
I really never used test kits because quite frankly, I would just EI dose and everything was fine. I am not really up on water chemistry because it has either been fine, or I test a bunch, read a bunch, and again, just can't figure it out so I change the water source.
I will buy a CA test kit ASAP and come back. I also realized they shut the water off today, either before or after I tested it out the tap, so I may need to test this over a few days and see what is going on, then weekly to see what is going on with my tests. I would be far from shocked if my water out the tap is changing, the building was build in 1974, has issues, is worked on all the time, and we have 200 units in the building so it is quite a large place. We also replaced the sewer and have consistently had clogs, even flooding in some units so who knows if they are dumping things in the water. I certainly don't drink out the tap here.