Hello; You are focused on the nitrates. That nitrates are a concern but with such high nitrates it seems possible there are other things in the water.
Hello; The 4 pothos are a start. You should be able to tell if more plants are needed by testing for the nitrates. My guess is that you will need to grow a fairly large mass of plant material to keep the nitrates at a decent level.
I have followed this and some of your other threads. It takes a while but eventually enough information has come thru. There are answers in the many posts members have made while trying to aid with your issues. I fear that you have too narrow a focus and may be overlooking the other issues.
First you have bad source water. How bad is unclear. We know of the nitrates and you are focused on that. If there are other things in the water then fixing the nitrates will help only that narrow issue and here is a potential for others to remain.
Next is that, so far, your approach to the nitrates appears too weak for the scale of the issue. My guess is that you will need a lot more plant mass to take up enough nitrates to matter. This is why the suggestion for a pre-treat tub was made.
Next you have too much fish mass for the water conditions. Maybe not too much for someone with good tap water but when starting out with high nitrates in the tap, like you have, then yes likely too much. Recall that the most common method for dealing with nitrates is a water change to get them down to below 20 ppm. Your tap water starts out at 40 ppm and then the fish add to that.
Lastly it may be that the focus needs to shift to some way to pre-treat your available tap water or get another source. Maybe an RO unit or some other such setup.
Good luck
